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Discussion Starter · #1 ·
I'm having major issues with my LT1. It would start and die and random, but would always start by third try. Then last year it would start and die, and sometimes it would run rough, really rough, then smooth out and run normal and start correctly for weeks on end. This year I have been driving it and one day it started and died and will not stop doing it. I can start it and get on the gas pedal and feather it to start. But it is running really rough. Smells like gas. It will run rough from 30 seconds with the RPMs being kept up then after 30 seconds it smooths out and runs great. But the starting is really rough, so I'm not driving it.

I have my laptop hooked up running a scan program. Once it starts, and I feather the gas, the code 18 - injector circuit pops up.

I have been back and forth, and upside down with the injector circuit. Here is what I have tried.

I put a noid light on the injectors and checked them while cranking, and while running. Here is what I found:
Number 4 and number 7 didn't blink right away. They would start blinking 30 seconds in after running. from what I read, the PCM checks the circuit on start up, if there is an issue with will re-try the signal after a period of time. I assume that is why it starts working after 30 seconds.

I pulled all the injectors. Tested the ohms, I had from 11.9 to 12.3 on all the injectors. I have a second used set of injectors. they all read 12.1 ohms. I swapped them in. no change.
I tested them and sprayed thru them before installing them in the car. They all worked.

I tested 12v to all the fuel injectors. I unwrapped all my wires and checked each wire all the way to the power source. No issues. I used a test bulb I set up with a 1157 bulb in it. I load tested the wires and was able to light the light bulb bright with the test on each injector.

I pulled the red connector off the PCM. I disconnected all the connectors off the injectors. I then load tested each PCM signal wire to the injectors. I ran 12v positive thru each wire to light the 1157 bulb. Each wire passed the load test by lighting the light bulb bright for over a minute.

I then swapped out my PCM with my spare PCM. No change. Still starts and dies.

I checked that all four grounds coming from the PCM where grounded and good. I will be load testing these also.

I cleaned my MAF sensor. When I pulled it, I thought I found the problem. The wire connector and 2 of the three wires bad. the signal wire was still good, but t he ground wire was barely hanging on, and the wire that goes to the O2 sensor was broken off. I replaced the connector and cleaned the MAF. No change.

I think I have tested or swapped out all the components in the injector circuit and can not find the problem.

I do not want to start throwing parts at this, as I've read previous threads from people with the same issue. They started throwing parts at it with nothing solved.

Other things I noticed on the scan. I compared it to a scan I did 5 years ago when it was running good.
-My MAP pressure is too high. Not running it reads 100kpa @ 4.75 volts. While running it goes from 70 to 100 kpa and the volts get down to about 3 volts. Is something wrong with the MAP sensor causing it to read high. I plan to vacuum test the MAP sensor today. it looks like in the old scan it would read 25-35 kpa.

My IAC readings are all over. They are at 160. I am going to try to clean this. My old scan the numbers were much lower.

I am not sure if the IAC and MAP readings are off from how bad it runs?

My fuel pressure is good at 40 psi when running. No fuel at all coming out of the regulator. Pump and filter where replaced last year.

I keep coming back to my DTC 18 being set.... I am not sure if these sensors would effect that?

I am at my wits end trying to figure this out. Anyone have any help? Thanks.
 

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Commendable all the tsting you've performed. You're already in the weeds, so the only thing I can add is the first thing comes to mind with the symptoms listed at the top have been those of many with their no-start-instant die-hard start-bad run of a failing/failed ECT and/or its connector.
 

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I am not sure I can help but would like to congratulate you on your methods of testing the wire harness.

I would suggest testing the injector's resistance from the PCM end. Maybe a connector at a injector is adding resistance. I would use this opportunity to wiggle and shake the wiring harness as well. The PCM can see a fault(open) that a light bulb may not show but a needle(analog) ohm meter may show the very fast open or high resistance the PCM is suggesting with DTC 18.

Is the idle air passage clean? IAC may be reacting to missing injector pulses. Memory says 40-60 is normal but please verify.

Any chance the PCM ground is a problem? (by the ICM/coil) You could also check for voltage drop while the PCM is on.

Have you tried running with the MAF unplugged?

Do the IAT and coolant temperature match before a cold start?

A amp clamp connected to a scope would show a waveform for four injectors at a time. If connected on the fuse side. You would see if one was functioning different and if one was not being commanded. You could also check each injector this way. This is a good test as the clamp requires no piercing of wires or back probing connectors.

I know the DCT points to the injectors but put a timing light on the coil wire and see if the pattern is even when the engine is running.
 

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Discussion Starter · #4 ·
Thanks!. I will go over the wiring to that also. Basically that is my plan tonight is to test each sensor to see if they are functioning correctly.

I will say that the temp reading while scanning was correct to the ambient air temp, and the ECT matched the intake temp. I will load test those wires also and check my sensor. I also have a whole spare engine, so I can swap out sensors to see if they make a difference.
 

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Discussion Starter · #5 ·
I am not sure I can help but would like to congratulate you on your methods of testing the wire harness.

I would suggest testing the injector's resistance from the PCM end. Maybe a connector at a injector is adding resistance. I would use this opportunity to wiggle and shake the wiring harness as well. The PCM can see a fault(open) that a light bulb may not show but a needle(analog) ohm meter may show the very fast open or high resistance the PCM is suggesting with DTC 18.
Thanks I will try that. That is a good suggestion, see it as the PCM would see it.

Is the idle air passage clean? IAC may be reacting to missing injector pulses. Memory says 40-60 is normal but please verify.
I will pull this, test the sensor and clean it out.

Any chance the PCM ground is a problem? (by the ICM/coil) You could also check for voltage drop while the PCM is on.
I am going to load test and trace the PCM ground tonight. I'll check these also. Thanks.

Have you tried running with the MAF unplugged?
Yes, check engine light comes on. It didn't go into limp mode.

Do the IAT and coolant temperature match before a cold start?
Yes

A amp clamp connected to a scope would show a waveform for four injectors at a time. If connected on the fuse side. You would see if one was functioning different and if one was not being commanded. You could also check each injector this way. This is a good test as the clamp requires no piercing of wires or back probing connectors.
This would be cool. I do not have a scope.

I know the DCT points to the injectors but put a timing light on the coil wire and see if the pattern is even when the engine is running.
I will check this also. I wasn't sure if this could be a spark issue, fuel issue, or what.... I was going to try each plug wire also. My number 4 cylinder exhaust on the header was cold on start up. But then kicks on after the 30 seconds....
 

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If you are using Ehack on your laptop I think it has a cylinder balance or compression check that shuts down one injector at a time. It might give you some info?
 

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The IAC is just a servo motor without a sensor. GM recommends not moving it by hand.
Wood Gas Auto part Nozzle Metal


The idle air goes through two paths. Yellow is the straight through so the throttle valves can be always closed. And the IAC path(red).
Automotive tire Motor vehicle Rim Alloy wheel Automotive design


Gas Bicycle part Rim Auto part Bicycle drivetrain part
 

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If you have a spare IAC you might try swapping it.

 

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Discussion Starter · #9 ·
If you are using Ehack on your laptop I think it has a cylinder balance or compression check that shuts down one injector at a time. It might give you some info?
Thanks I will look into that. That would be handy for testing, and narrowing it down to which cylinder is having the problem. I am currently using scan9495 to scan it.
 

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Discussion Starter · #10 ·
If you have a spare IAC you might try swapping it.

Thanks for the picture and the links! I think I am already using my spare IAC. I will check and look at the testing procedure of it. Thanks!
 

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Discussion Starter · #11 ·
If you are using Ehack on your laptop I think it has a cylinder balance or compression check that shuts down one injector at a time. It might give you some info?
Nevermind, I found it. EEHack!


Do you have any info on EHack? A web link or? I did a few searches but I am not coming up with anything. Thanks.
 

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I am a firm believer in having a good cable not a ebay chip with leads jammed into the OBD.

Steve"s website:

Too much information:

It started as a PCM tuning package but tuning, uploading, and diagnostics seem separate now. I have not loaded or used it. Google it a bit, other forums have some info.

The three things you may be interested in are injector shut down, egr valve command, and forced open loop.
 

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Discussion Starter · #13 ·
I load tested my grounds to the PCM. They all came out good.

I checked my fuel injector ohms at the PCM connector. All were 12.3-12.5 with # being 12.0.

I used the $EEhack program. It worked good. But I notice it shows no DTC's in this program, yet 9495scan shows DTC 18.



Still all the same issues happening. Noticing some back firing happening now. I am wondering if this will be an opti issue?

I don't know what do do next....
I ordered:
opti
new/rebuilt fuel injectors
ICM
IAC
MAP
Spark plug wires....

I plan to try one at a time and see what happens....

Is there a section in the forum to upload scan data for someone to evaluate? I haven't been on the site in many years! Maybe someone would see an issue right off the bat from the data.
 

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That MAP value is definitely wrong; I usually see a 35kPa at idle on a stock B-body LT1. Does the MAP value change after the other cylinders kick in? MAP sensors rarely fail but if it's cracked, that may be allowing a leak into the sensor (just a wild guess here). All those parts you got, if you're patient, only swap in one at a time to see what changes. I'd start with MAP. Also check for vacuum leaks. Has the intake manifold been off? Perhaps a leak at the intake manifold gaskets on one or two cylinders (also a long shot).
 

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Discussion Starter · #15 ·
Thanks! Yes I agree they MAP is off. I got it running last night and let it run till it smoothed out. I started logging at that point. The MAP calmed down at was in the 30-40 range. So that is good. The IAC also calmed down and I could see it adjust when I gave it gas.

It's seems to be running bad now, and getting worse. I even had a backfire or two. Cylinders 3 - 4 - 6 the header was cold on those at start up. I think I have a miss going on. Still getting the DTC18. I don't know if something else can trigger that code besides the injector wiring. I've been all over the wiring and it seems good. Even swapped out the injectors for an ohm box to simulate the injector and I still get the codes.

I pulled the opti off last night. I opened it and it was dry as could be in there. But I noticed what I believe is arcing on the cap. Did the opti's have issues arcing thru the cap? the arc path pretty much followed the curve of the wire that was on the other side. I could see the wire thru the epoxy. Basically in the middle of the path or the rotor spinning. The opti is not too old, but that doesn't mean anything.

Is there anyway to log a cylinder miss firing?

I get new spark plug wires tomorrow, so I will button this back together this weekend to see if it makes a difference. I sure hope so....
 

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Discussion Starter · #17 ·
My last one would have been AC Delco. The one I just bought is also a AC Delco. I see this one is different than my last one. My last one was bought roughly 8 years ago. The new one I see they added a wire extender and heat shrink off the the wire connection.
 

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My last one would have been AC Delco. The one I just bought is also a AC Delco. I see this one is different than my last one. My last one was bought roughly 8 years ago. The new one I see they added a wire extender and heat shrink off the the wire connection.
That is the newest GM remanufactured Delco Opti so at least you got the best one and not a cheap aftermarket one.
 

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Probably time to replace the Opti-Spark.
Don't forget to either make sure the Opti- vent & vacuum harness works, or replace it.
 

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I pulled the opti off last night. I opened it and it was dry as could be in there. But I noticed what I believe is arcing on the cap. Did the opti's have issues arcing thru the cap? the arc path pretty much followed the curve of the wire that was on the other side. I could see the wire thru the epoxy. Basically in the middle of the path or the rotor spinning. The opti is not too old, but that doesn't mean anything.
Yes, that's why they redesigned it with a vent. That flimsy little check valve in the vent line has a tendency to stick shut, causing a buildup that favors arcing. You can actually buy the check valves by themselves off of amazon, although I don't know offhand what hose size they need to be.

 
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