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Discussion Starter #1
I had a chronic oil leak in my 96SS (86k) that my mechanic told me was not in urgent need of addressing. Apparently replacing the seal would require removal of the transmission and other labor-intensive work. The car ran fine, but I eventually had the seal replaced (while the car still ran fine) a few weeks ago. Here is what was performed:

Initial Trip To The Mechanic
Serviced Rear Crankshaft Seal. This included removing the driveline, removing the transmission, removing the flywheel, removing the faulty seal, Inspecting the seal bore and crankshaft for wear, installing the new seal.

Drained Oil, Unbolted Engine Mounts, Raised Engine / Removed the Oil Pan & Cleaned Pan and Gasket Surface, Cleaned All Residual Oil From Engine Block, Cleaned Oil Pan Mounting Surface On Engine, Reinstalled The Oil Pan -Reinstalling the oil pickup tube as needed with new gaskets and seals, Lowered and Rebolted Engine, Oil Change.


Ever since then, the CEL has been on and there has been a misfire on #4, and there has been a strong smell of gas fumes. Compression tests are all good. It was noticed that the water pump was leaking, so a new water pump and opti was assumed to be the fix. Since then, I've spung for:

AC Delco Water Pump & Gasket
New Opti
AC Delco Wire Kit, SPLG
AC Delco Spark Plugs
Thermostat w/ Gasket
Tee Connection
AC Delco Fuel Injector Seal Kit
Fuel Injector Cleaning/Overhaul

The misfire on #4 persists. And, ever since the parts I mention directly above were replaced, the CEL flashes almost all the time. I should note that the first opti replacement didn't fix the issue so it was replaced again with yet another opti, in case the first replacement was a bad opti. These were purchased through a Chevy dealership and are OEM.

Another thing to note, and I hadn't thought to tell my mechanic this: before I took my car in for the oil leak, I would have to prime the pump in order to get the car to start smoothly. I stated above that the car ran fine, and it did. I would just need to prime the pump for one second before starting the car. I have done some reading on the forum today and it seems that a fuel pump and/or filter issue could play a part. I would expect my mechanic to have checked this possibility already, but I will ask him later today.

He advised me to drive it on the freeway a bit, in case there was carbon buildup that needed to be burned off, so last night I had it up to 4000 RPM (got the car up to around 110mph) in small spurts, and drove for about an hour straight at an average of about 70mph (maybe 2000-2500 RPM for the most part). The temp reading never rose much above the 1/4 mark the entire time, so the engine didn't seem to be very hot.

The issue still persists. My mechanic has given up and has advised me to take it to a machinist with engine expertise.

Can anyone offer any thoughts as to what might be the cause of the #4 misfire?
 

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Discussion Starter #2
I have been reading posts in other threads. Perhaps the O2 sensors could be the cause?

ok so i figured out the 96 ss i was working on. turns out that the 2 right side o2 sensor harnesses were swapped, so the front o2 was actually reading the rear and vise versa. the connectors are both the same, and they will reach either position and the wire colors are all the same too.
Or, perhaps it is the Crankshaft Position Sensor?

My brother has a 96 caprice wagon that has had a localized misfire on cyl 4 (constant) and 6 (intermittent) for a long time. I posted in the misfire thread with everything that we changed looking for it. NO CEL (except blinking when misfiring).

So after squandering countless hours and money looking at the ignition and fuel sources, it turns out it was the CKP sensor. WTF ?

My understanding was that the CKP sensor is basically worthless: The crank position sensor was added in 1996, for the sole purpose of meeting the OBD-II requirement of identifying misfires. IT IS NOT USED TO RUN THE ENGINE. All the 94-95 LT1s run w/o it. Numerous people have deleted the CKP sensor from the OBD-II cars, and they run just fine. The PCM only needs the cam position data from the optical cam position sensor in the Optispark. The PCM uses that data to set ignition and injector timing. It does not need CKP data, unlike other engines that use a traditional crankshaft position sensor. You can NOT tune an LT1 with only a CRANK position signal. In fact it can't run on the crank position, because the 94-97 engines use sequential injection. To sequentially fire the injectors, you need a CAM position signal. The cam position signal comes from the optical sensor in the Optispark - the low resolution pulse signal. Once you know the cam position, you also know the crank position, but not vice-versa.



So I have always ASSUMED that an LT1 CKP sensor failure could NOT affect performance. You SHOULD run perfectly fine without a crank sensor on an LT1, but that would set a code for the sensor. My brother had no code.

Well, the CKP sensor on the car was cracked. Replacing it (check the price on RMS... you'll fall on your ass) fixed the issue. I can't explain how or why, unless it was somehow shorting or grounding back to the low reference pulse circuit in the opti.
Does anyone have any words of wisdom?
 

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Hook the car up and learn what it is trying to tell you. Fuel smell is (obviously) unburned fuel, so chances are its not an injector, but it could be. What do the logs say about the fuel mix? One side (#4 side) should be pig-rich because the injector is pushing fuel that's not being lit off. That'll at least point you in one direction. Get a noid light and ensure all of your injectors are firing at the harness. You stated its #4. Pull the injector rail and pressurize it. Look for a leaking injector. Ohm #4 plug lead. Check all of your grounds. Pull the plug or use a tester and ensure the lead is actually getting the ignition signal to ignite the spark.

Three things are required: air, fuel, spark. One of those is missing.
 

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I have to believe there was something damaged during the original repair. The work done since then may mask the damage, but I would inspect the engine and transmission (everything that was moved). Atlantadan is right, I would diagnose before throwing any more parts at it.
 

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QUOTE: "The issue still persists. My mechanic has given up and has advised me to take it to a machinist with engine expertise."

wtf - Why a machinist?

Don't spend another dime on parts or labor until you have a proper DIAGNOSIS. As said above, good stuff.

Who is your mechanic? A qualified Indy or a shade tree type? You don't show your location (Mods - why isn't this mandatory?), but if you live in an area with a good selection of shops - shop around. Perhaps you will find a GM dealer or someone who has actually worked on an LT1 car. Your car is OBDII which opens up a world of diagnostics like fuel trims and all that good stuff. Spend your time finding a good shop, perhaps a shop that does diagnostics only that isn't out to sell you parts, ask questions before spending $$$. I hate to wonder what you have already spent.

2 new optis?? Did you get your old one back? FWIW - OEM optis are not the same as an original opti. Did they install new opti, water pump, and crank seals? Cheap parts, easy labor. Not listed above. In fact, they should have installed a new timing cover gasket since removing the oil pan is necessary for that (unlike a real Pontiac engine where it is not needed - loved my Ponchos!). (y)

It's too late now, but I wonder if your "mechanic" actually ID'ed the leak. These are known leakers at the intake, valve covers, and oil filter mounting which all run down to the pan and rear seal - which probably was leaking also. But was it bad enough to require replacement?

And 4000 rpm - 110 mph with a misfire, ain't good.

Later, I'm out.
 

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Discussion Starter #6
I thought the initial post was long enough already, so I didn't go into too much detail. But for those who are concerned about costs, yes, this is becoming rather costly; however, the mechanic has not charged me any labor for the engine issue, and he has spent many hours on it. My car was there for two weeks straight. He even paid the local Chevy dealer to send out one of their own to help diagnose the issue, he says. Community Chevrolet (Burbank, CA) is quite close; I do believe him when he says he did so. The mechanic still wasn't able to discern the problem after all that time and work, even with the help from the Chevy tech, so he told me I didn't have to pay him at all (I have been going to this shop for a few years). But I didn't want to drive away with parts I didn't pay for, so I did pay for the parts. It stung, but it was the right thing to do.

I have read other threads describing similar circumstances when it comes to misfires. Diagnosing the source of a misfire is surely not as cut and dry as we all want it to be. From what I've read on these boards I don't think it was wrong of my mechanic to assume that, given a leaking water pump, the original opti might have been compromised. If I were more savvy about car mechanics, I'd be very willing to take the time to diagnose the problem myself. But I don't want to further break anything while getting overconfident underneath the hood at this point. I know too little.

@nickss96 agreed that something must have occurred during the crankshaft seal replacement that caused the misfire. I started this thread because my mechanic has seemingly spent a lot of time trying to figure out the problem, and I'm no expert, so I thought I'd reach out to the experts. What could have gone wrong during a crankshaft seal replacement that would cause a P0304? Maybe the O2 sensors were disconnected in the process and when they were reconnected, they were inadvertently swapped front/rear? Or, maybe the crankshaft sensor was broken in the process of replacing the seal? What do you guys think? I want to gather a list of possibilities so I can take it back to my mechanic and make sure they're all addressed, before I take my car somewhere else and have to pay them to start all over from square one. Thank you for the thoughtful replies. @grandpas wagon you seem a bit worked up, but I do appreciate your concern. Given my limited knowledge of car mechanics, I think I'm following a reasonable course of action. Any expertise I can receive from these boards is a blessing.

@atlantadan thank you for the reply regarding the injectors. If there is a job I can try on my own without any experience, the injectors seem like a worthy place to start. The mechanic is certain that the wires are all good, but I don't know exactly what diagnostics he has performed on them to come to this conclusion. I do wonder whether a noid light might tell a different story. It's very valuable advice. Thank you.
 

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Discussion Starter #7 (Edited)
Don't spend another dime on parts or labor until you have a proper DIAGNOSIS. As said above, good stuff.

Who is your mechanic? A qualified Indy or a shade tree type? You don't show your location (Mods - why isn't this mandatory?), but if you live in an area with a good selection of shops - shop around. Perhaps you will find a GM dealer or someone who has actually worked on an LT1 car. Your car is OBDII which opens up a world of diagnostics like fuel trims and all that good stuff. Spend your time finding a good shop, perhaps a shop that does diagnostics only that isn't out to sell you parts, ask questions before spending $$$. I hate to wonder what you have already spent.

2 new optis?? Did you get your old one back? FWIW - OEM optis are not the same as an original opti. Did they install new opti, water pump, and crank seals? Cheap parts, easy labor. Not listed above. In fact, they should have installed a new timing cover gasket since removing the oil pan is necessary for that (unlike a real Pontiac engine where it is not needed - loved my Ponchos!). (y)

It's too late now, but I wonder if your "mechanic" actually ID'ed the leak. These are known leakers at the intake, valve covers, and oil filter mounting which all run down to the pan and rear seal - which probably was leaking also. But was it bad enough to require replacement?

I could write more but my blood pressure is elevated when I think about this.

Later, I'm out.
I should answer some of these questions.

Who is your mechanic? A qualified Indy or a shade tree type?
I'll PM you

2 new optis?? Did you get your old one back? FWIW - OEM optis are not the same as an original opti. Did they install new opti, water pump, and crank seals?
I bought the first one myself from a Chevy dealer in Orange County. When it didn't solve the problem the mechanic traded it in for a second one within the dealer network. I did get the "core". Should I keep the core?

New AC Delco water pump. I don't know about the crank seals but I would assume they did(?)

It's too late now, but I wonder if your "mechanic" actually ID'ed the leak.
Regarding the leak, I'm inclined to believe he was correct. It has had various leaks over the years and I had held out on this one for a couple of years while he fixed the others.
 

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Discussion Starter #9
Just to clarify the seals. That is a water pump seal, an opti seal, and a crank seal.

Thank you for the link. I'll find out whether the shop installed new ones before I order. But I think it does make sense to have new seals. And, since you say that the original opti is superior to the one I bought from the dealer (re-manufactured AC Delco) perhaps I should put the original opti back, and the new seals in at that time.
 

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Who is your mechanic? A qualified Indy or a shade tree type?
I'll PM you
Don't PM info like this, people are trying to help you. Post it here. We don't need the NAME of the shop, just the type of show so we know what you're dealing with.

Your car is a 1996. Start with plugging in a scanner, get the codes, and post them here.
 

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Discussion Starter #11
Don't PM info like this, people are trying to help you. Post it here. We don't need the NAME of the shop, just the type of show so we know what you're dealing with.

Your car is a 1996. Start with plugging in a scanner, get the codes, and post them here.
I don't want to turn this into a witch hunt about the mechanic. I do think the issue began because of something that occurred during the seal replacement, to be clear, but I do also believe them when they say that the cause is so illusive that the Chevy mechanic they called over to help them with the issue couldn't even figure it out ...and they paid him for it (and didn't charge me).

They have over 250 reviews on Yelp and 4.5 stars. Here is what their homepage mentions:

The shop uses ASE-Certified mechanics
CarFax 2019 Top-Rated Service Shop
Recognized in the Burbank Business Hall of Fame for being selected for seven years running as the Best of Burbank for Automobile Repair (most recently in 2020)
Angie's List Super Service Award (I see that their last one was 2016. They show five)

I'll see what codes it throws other than the P0304 and follow up.
 

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I'm just back from Pep Boys (not my mechanic) and they said all it's throwing is a 300. They didn't see a 304. No other codes than a 300. This is news, because my mechanic has been calling it a 304 for weeks. So, the #4 misfire has now become a random misfire.

Strong gas fumes. The idle was slightly better on the way home. One thing: when I would stop at a red on the way there, the car would almost-almost want to stall, but then it would catch itself and surge a bit to find its equilibrium, and then idle all right. A bit rough, but all right. On the way home, I stopped at a couple of reds and this low idle/surge never occurred. But strong gas fumes, and still rough, of course.
 

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Isn't there some kind of sniffing device that can pinpoint a fuel vapor source?
 

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One thing I do when I'm troubleshooting something is to google it. 'LT1 random misfire' gives links to several forums and how the problem was tracked down. There is also a video that shows how to use the noid light:
 

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Equip yourself with a good OBD2 reader/logger. It will save you money and time and make DIY a lot easier. Palmer has a solid OBD2 product.

A random misfire is a different animal entirely. It points to either an overall fuel delivery problem or an overall ignition problem. I would guess it is ignition.
  • Check fuel pressure. Key on engine off and key on engine running. Then both again with fuel pressure regulator unplugged. Check against specs. Too much or too little fuel will cause random misfires.
  • Check for fuel in the fuel pressure regulator.
  • Datalog the car and pay attention to fuel trims and O2 readings in the log. If the problem is isolated to one side, that will tell you which side. If the problem is isolated to pre or post closed-loop, that's valuable info. As is the window when the misfires happen. All of that will be readily apparent in a datalog. It'll also tell you if the O2's are functioning correctly.
  • Check your grounds at the driver's side head, at the coil stud.
  • Check the ground on the back of the engine at the firewall.
  • Ohm out the coil. A bad opti or bad grounds can damage or completely wipe out the coil.
  • Ohm out and continuity test ALL of the leads going to the coil and the opti. Proper specs are on Shbox.com
 
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Discussion Starter #16
Equip yourself with a good OBD2 reader/logger. It will save you money and time and make DIY a lot easier. Palmer has a solid OBD2 product.

A random misfire is a different animal entirely. It points to either an overall fuel delivery problem or an overall ignition problem. I would guess it is ignition.
  • Check fuel pressure. Key on engine off and key on engine running. Then both again with fuel pressure regulator unplugged. Check against specs. Too much or too little fuel will cause random misfires.
  • Check for fuel in the fuel pressure regulator.
  • Datalog the car and pay attention to fuel trims and O2 readings in the log. If the problem is isolated to one side, that will tell you which side. If the problem is isolated to pre or post closed-loop, that's valuable info. As is the window when the misfires happen. All of that will be readily apparent in a datalog. It'll also tell you if the O2's are functioning correctly.
  • Check your grounds at the driver's side head, at the coil stud.
  • Check the ground on the back of the engine at the firewall.
  • Ohm out the coil. A bad opti or bad grounds can damage or completely wipe out the coil.
  • Ohm out and continuity test ALL of the leads going to the coil and the opti. Proper specs are on Shbox.com
I have been in contact with the mechanic today and he's not interested in taking the car back. So, I'm back at square one with another mechanic. I think it's fair to take a pause here and let those with advice to offer sit back until I have news from the next shop I go to. I will point them to this thread if/when it seems appropriate.

@atlantadan thank you for the wisdom. I have ordered the Helm manuals and an OBDII bluetooth adapter. I'm going to try to be more self-sufficient going forward. I don't have any diagnostic tools, but I have the typical starter stuff (ratchet, etc) that should enable me to get my hands dirty and go from there. I hope to at least be able to present any future issues with a better understanding of my car, and perhaps I can even do the repairs myself. Surely many would agree that it's becoming ever-more the case that if you want something done right, you've got to do it yourself. I'd be happy to fix my car if I could. Might as well take some steps in that direction. ...I just think diagnosing an issue that even a Chevy mechanic couldn't fix might be too difficult a task, right out the gate. I do appreciate those who have chimed in and I hope to be back soon with news.
 

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I did have a misfire after changing plugs and wires... ended up being caused by the metal heat shield on the spark plug boot. See if you can notice any arcing when running... I was able to see it in the garage with the lights out and the garage door cracked slightly (for exhaust fumes). Removed the heat shield and used an insulated spark plug wire/boot sleeve and no more misfire.
 

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Look on the fuel pressure regular vacuum line an trace it to the intake manifold. Where does it enter, see if next to number four intake runner. Reason I had an issue on my 95 where 5&6 plugs were black an miss firing. After replacing the cap and rotor, IAC motor, wires, plugs, still had the issue. My son had told me to check the fuel pressure regular some time ago, this after all most a year figuring out what the heck. Well I did pull the vacuum line the regulator and car started to run great, when I looked to put the line back, yah a nice stream of gas pouring out of the regular vacuum port. Just a suggestion.
 

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Discussion Starter #19
I just spoke with the new mechanic. He's had the car for three days and has come back with an answer. He says the fuel injectors are going out, and that cylinder 4 is the worst, but there are three others that are also misfiring to a lesser degree. He's advised me to replace all eight, and mentioned that the fuel regulator might also be bad, but he's 100% sure that the injectors are the greatest contributing factor. Looking at about $1800 after tax for AC Delco injectors plus $300 labor. Thoughts?
 

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Or similar service.

It's your money.

I'm sure others will recommend other injectors or service.
 
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