My guess is either wiring at fuel pump is heating up and voltage dropping (not uncommon wiring fail) or fuel pump itself is failing after extended use given it drops the longer engine/car is running

 

Thanks for your reply. Are you speaking of external wiring or in tank wiring? Once running, it stays at 36psi, except for revving spikes.

 

I would consider the fuel pressure regulator is sticking.

If you remove the vacuum line and it smells of gasoline the fuel regulator is bad.

Started having some slightly rough idle, and surging or hesitation at low speed.

If the fuel pressure is steady at idle and during hesitation how is it causing the issue?

Tape the gauge to the windshield and watch the pressure under normal running conditions.

Wide open throttle without engine load is not a great test of fuel pressure as the vacuum that controls fuel pressure changes a lot.

 

Thanks for the reply. No smell of gas or gas leakage at the FPR vacuum line, But i have heard of them being intermittent. I'll pull it and make sure the screen is clean and then run with the Tester on the windshield. See if it changes results. Any thoughts or experience with other possibilities? The opti looks clean, new waterpump 3 years ago and no leaks or oil drips from the front. I vacuum tested the opti harness and it was right on with the numbers.

 

I am not a engine expert.

My first move it to plug in my scanner.
Eehack is free and has some great features.
I check fuel trims and that the idle air control numbers are normal.
Before a "cold start" I compare manifold and coolant temperatures (they should be close).
Check again at running temperature that they make sense.
I will check that the engine will go into closed loop.
The O2 sensors may not affect much at idle but at running temperature left and right should be reacting the same while driving.

I will clamp a timing light on each plug wire in case the plug or wire is so bad the timing light is blinking differently (sporadically).
A spray bottle of water can be used to spray the finest mist possible around the spark plug wires to check for arching.
The darker the better for this test.

I use a unlit propane torch to check for vacuum leaks around the intake manifold.

Back to the fuel pressure.
Unplug the vacuum to the fuel regulator and block the vacuum line.
I expect your key on pressure to be close to your engine running pressure.
Even revving the engine should not make a big change to FP.
Without the fuel regulator the engine will run differently.
Reconnect the vacuum to the FPR before driving.

Stumble can be very hard to figure out. The PCM is trying to prevent it.
A non computer car would show the symptoms better because it cannot change air, fuel and timing to hide the problem

 

Excellent suggestions, thank you. I currently have been using a AutoXray EZ-Scan 3000 to check for something out of wack. Though I have used a HP Tuner on my last custom build long ago, I have forgotten most everything about it. And its OBDII. Came across this as a cheaper Tech1 to use. Here are a couple screen shots of it at Idle warmed up. I will do a scan with FP Tester taped to window and capture a log. Since Im not proficient at all anymore with tuner software, does anything here stand out with this small capture? Anything I should focus on for the next scan? (Currently away and wont be able to scan til I get back) One thing I have noticed is that on cold start ups, it will idle high, around 1025rpms and wont idle down once warmed up. Restarting it drops it back down to around 675rpms.

 

One thing I have noticed is that on cold start ups, it will idle high, around 1025rpms and wont idle down once warmed up. Restarting it drops it back down to around 675rpms.

Look at loop, coolant temp, commanded rpm, rpm, Park/Neutral, TPS,and IAC .
When it is high idle and again after restart.

I do not see any bad numbers in your screen shots but I am not the best person to ask.

 

If fuel pressure is fluctuating, assuming your gauge is accurate, than various data a scan tool like STFT, misfire, etc will also show variances. The fact a FP gauge is showing variable fuel pressure points to some element (part of wiring) of fuel system is intermittent/faulty. The associated scan data will just echo "something" is not right

 

One thing, may or may not be your high idle rpm, is a sticking IAC. You would need to remove it and spray with throttle body cleaner and the orifice it goes in. Do not push the end of the IAC

 

I just looked at all of your scan screenshots. Everything looks normal. Your fuel pressure is fine. Intake manifold vacuum I'm guessing is fine since the MAP voltage is so low (1.23V). The block learn values are below 128 (117 and 123) which means the PCM is pulling a little bit of fuel at idle. That tells me the O2s are working right and you don't have any vacuum leaks or exhaust leaks.

Try unplugging the MAF and drive it for a few days and see if the stumble changes. You may have a soft failure of the MAF; I've seen this a few times.

High idle could be a sticking IAC but you can rule that out by cold starting it and then watching commanded IAC position with a scan tool. If it doesn't drop, it's a bug in the PCM; I've seen the behavior you describe once before; I never did get to the bottom of it (didn't have time; just restarted the car and kept going). You can also use the Controls menu in EEHack to force a particular idle RPM or IAC position; that may help you in diagnosis.

Back to the stumble: you may have clogged or failing injectors, or a bad spot in your TPS, but with such low miles I'd lean towards the injectors. Run a bottle of injector cleaner through a tank of gas, and see if it changes anything. I sell better injectors for these cars at a cheaper price than you can replace them for anywhere else; info is in my signature at the bottom of this post.

 

Thank you all! Great people on here. I appreciate all your input. I will have a chance to try your suggestions in the next day or two when I get back from traveling. Will keep you posted on results. Any particular injector cleaner you recommended over others?

 

EEhack's cylinder balance test may show a pintle or bad spray problem with the injectors.
It is rare eight injectors fail exactly the same way.

A oscilloscope can show both electronic and physical problems with a injector.

If one injector is bad personally I would change all due to age and low use.

OP could also monitor the MAF if the scanner can graph or display fast.

 

@sealclubber I edited my post #11 with new information and a bit more clarity. While you are traveling, buy one of those ALDL cables so it can be at your house when you return.

@Z09B4U the last MAF failure I experienced on a guy's car, the EEHack data looked reasonable. It wasn't until we unplugged it and the car ran perfect that we realized the MAF was the cause of the stumble.

 

OK, back now. I will be trying everyones suggestions as tests. I really appreciate all your input. So starting with the easiest that everyone has touched on is the IAC. As I have seen many times said, the lower screw is a bear but using a flexible gooseneck drive, it wasn't too bad. Once out is was dirty with carbon and soot. So, I took a hammer to the pintle and repeatedly smash it till it succumbed to my dominance over it and complied with my demands!

Kidding! Of course. Bet you woke up with that one..lol. I sprayed it with MAF cleaner since I didn't have any TB Cleaner on hand, while gently but firmly holding the pintle from moving. The carbon was pretty bad so dribbled some brake cleaner on the metal plunger only, while lightly rubbing with clean paper towels and a toothbrush. Got most of it but still some nasty crust on it. Using "0000" Steel wool that I use for watches that I clean and repair, I gently polished it down without scratching it. Did the same with the IAC cavity, (thats a before pic). Put it in and started it up.

Felt the difference immediately! Rpms went 850 instead of 1025 and slowly descended as the engine warmed up, (which it wouldnt do before, just revved high). Dropped down to 675 rpms and took it for a drive. A little hesitation and surge at first but it settled in nicely. Did 10 miles and shut it off. Took it for another cruise and the PCM seemed to have started to adjust. It was smooth, powerful and just glided down the road like a Roadmaster should. Smoothe and quiet is what I like for this car as its my highway ride for long distance. I have hotrods for around town. It is back and I could not be more pleased that it feels like it just came off the showroom floor, if not better! Threw some Injector cleaner in the tank and called it a night.

Will be trying everyone elses other suggestions in the next few days, so stay tuned.

Again, I can't thank you all and this forum enough for being so kind and helpful. 👍 Youre the best!!

Quick side note I did notice that it keeps showing temps at 209 -219 degrees..? When I hit the thermostat housing with my Temp gun it shows 175. What is the proper Temp? 195? I do have the lighter Lesabre clutch fan mod on mine, its the Tow Package option. Think it has anything to do with that?

 

The thermostat housing will always read the lowest temperature on the engine because that's where radiator return coolant enters. 175 is normal range. When you say 209-219, is that reading from the climate control, or the instrument cluster?

If your IAC was that dirty, your throttle body is probably dirty too.

 

Thanks. I'm reading it from the Scan through the OBD1 connector. Its on the first pic of the AutoXRay scanner. Funny you should say that, as that was the next thing to do this morning, was to clean the TB and MAF. The TB wasnt that bad with a visual inspection, little at the bottom of the Butterflys, but was still sealing tight and the passage down to the IAC looked clean but hit that with cleaner too.

 

Open the butterflies and look in; the butterfly edges, rears, and bores need to be cleaned semi-regularly. Also, make sure the fixed bleed hole in the lower center between the butterflies is clean and clear; that's "base air" - enough to keep the car running at 550rpm even if the IAC is completely closed. But when that hole gets gunked up, the idle gets more dependent on the IAC, and is likely to fluctuate or oscillate as a result.

Temp: 209-219; was that measured while idling and stopped? How fast is the mechanical fan spinning at idle? Is the lower air dam and all radiator air deflectors still in place?

 

You might want to find a gasket set and clean the PCV section. It can get clogged up.

I find removal and full cleaning of any throttle body better than on car sprays.

 

Agree TB cleaning, as well as IAC, best done with TB removed. Its the cavity the IAC goes in that needs cleaning also

 

Thanks for the reply and pictures! Excellent advice. My TB looks like yours but a little cleaner at the Butterflies, and holes are clean but I will pull it and be complete. I have heard the gaskets from some suppliers arent exact for the PCV cover. Any suggestions for a quality gasket set?

My temps are around 209 running, while the high is idling, usually around 212. That 219 was last night, highest Ive seen. The blades spin nice, the antifreeze and rad was flushed 2 years ago with light use. Wouldnt hurt to do again I suppose.

I did pull the MAF and gently cleaned it some with CRC MAF cleaner. Let it dry for 2 hours and reinstalled. As expected, the car acted as before the IAC cleaning, but I expected that the computer needed to settle from the MAF cleaning and ran 10 miles, town and highway, 3 times, shutting it off and restarting the car. Each time it got smoother and settled in, running great in the end. I usually like to let things adjust before trying anything new so as not muddy the waters with what did what, but...

I'll run it on the highway for 2 hours for a trip tomorrow and see how it holds up. I also ordered the Cable today, be here in a few days

 

There are a couple of things to keep in mind with the IAC.
There is no feed back in the system.
It should be calibrated.

https://www.impalassforum.com/threads/idle-air-control-iac-valve-relearn-procedure.1324987/

The PCM moves the ICM in steps. If the pintle does not move the PCM does not know.
So when the pintle does not move the IAC at idle numbers do not look normal and will change every time the pintle is stuck and skips a step.

I would monitor the IAC at closed loop idle a few times even if things have improved.
See post #9.

The IAC is active all the time. If it is like most engines it will count up a bit with increased throttle so it can gently return to idle.
In some cases if the pintle does not have full travel it will affect the engine under load while driving.

I have heard the gaskets from some suppliers arent exact for the PCV cover. Any suggestions for a quality gasket set?

Check your local parts stores, in person when they are not busy, as well as the internet.
You can ask about the Camaro and Corvette (95-97????)

I leave the coolant section alone, and you only need the PVC gasket if it is already aftermarket or someone has used carburetor cleaner or brake clean on it

cator