LT1 Based Engine Tech 1993-1997 LT1/LT4 Engine Related

Erratic idle bad WOT power

Old 02-14-2019, 07:39 PM
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Erratic idle bad WOT power

inhave a 94 Buick RoadMaster wagon. It the low power LT1 motor and 4L60. The car has the trailer package rear end but itís a dismal 2.92. The car only has 110k on it and no winters so itís nice.

Anyway, I had a new fuel pump and fuel filter installed. Then we changed oil,filter, air cleaner, and belts. Less than 500 miles later the Opti failed
. I then had a new old stock GM Opti and water pump installed. This Opti also has the vacuum line as does my car. This was replaced with new stuff.

The car runs nice and gets reasonable mpg.

Then I went to pass a car and it just sputtered on the downshift. So I tried it by manually shifting to 3rd or direct. It starts moving for a second os so then just breaks up.

Along with this the idle is absurd. It idles fine then will speed up to about 1200 rpm. Then goes back to normal idle. You can imagine trying to get into a parking stall with this. Itís icy and slippery now and itís a real trick to drive this car.

I tried to isolate when it does this idle up. Itís completely random. The motor can be hot or cold, doesnít matter. It doesnít matter if you hold the brakes hard or just easy. At idle with firm brakes it will buck and almost feels like a miss but itís quick like one misfire.

today the CEL came on and stayed on for 50 miles. Then on the way home it was off. It usually comes on for a couple minutes in the morning but not every morning.

I know this is a big messy but it about the way the car runs. Iím a bit disappointed as these are supposed to be ďold guysĒ cars. Jump in it and head for the golf course. LOL Easier to call UBER.

this is OBD 1 so not every one still has this stuff.. the guy that usually takes care of me has had his high dollar unit crap out twice while testing this car. Iím bugging him but Iíd like maybe some first hand things to look at.

i did check over the air intake to the throttle body and itís in good shape with no rips or holes and clams are tight.

any help would be appreciated.

Byron
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Old 02-15-2019, 08:56 PM
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Re: Erratic idle bad WOT power

You need to read the codes. Then run a data log.

https://www.camaroz28.com/forums/com...95-lt1-874306/

I can help interpret the data log.


After replacing the fuel pump and filter, have you checked the fuel pressure - 1) at prime, watching how fast the pressure drops after the pump shuts off; 2) at idle, with and without the vacuum compensation line connected to the fuel pressure regulator; 3) then at WOT above 4,000 RPM.


Add a ďsignatureĒ per the suggested format:

https://www.camaroz28.com/forums/lt1...nature-879835/
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Old 02-15-2019, 09:25 PM
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Re: Erratic idle bad WOT power

Hey thanks for the comments. I no longer have a good place to work on my cars so I’m at the mercy of a really good guy. He is very busy so scheduling a day with him is tough. The last time his high dollar scanner pooped out. I’m just trying to fix what I can before I get back in with him. My car is OBD 1 so not everyone has a good scanner. His was just starting to show things and it black screened.

Ill fill in a sig. sorry.
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Old 02-16-2019, 09:18 AM
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Re: Erratic idle bad WOT power

Guess you didn’t click on the first link. If you take the time to click on the link I posted, it takes you to information on Scan9495. This is FREE software, specifically written for the OBD-1 LT1 engine/computer, that you can download to a laptop, and use to pull codes and run a data log that records sensor readings, actuator instructions, and PCM parameters. All you do is buy or build a cable, plug it into the DLC connector under the dash, and drive the car. Does not require a place to work on your car.

After you run the log, you export it as a .csv file, post it here, and we can go through the log looking for the cause of the problem you describe. Sometimes we find the problem, sometimes we don't. The author of Scan9495 (GaryDoug) is a member here, and available to address any issues with the hardware, software install, or data logging. He has actually built a physical simulator of the LT1 control system, including a working PCM, Opti, sensors, etc. A graduate electrical engineer can be a huge asset in sorting the problems out.

I've been using scanners and data logs to solve LT1 problems for 20+ years, going back to Diacom, Scanmaster LT1, TTS DataMaster, etc. If you poke around in the “sticky” posts on the “Computer Diagnostics & Tuning” forum, you'll find a link to my 20+ page guide to interpreting scanner info.

From my experience, spending the money for a cable is a lot less expensive, and a lot more productive than playing parts replacement roulette.
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Old 02-16-2019, 11:58 AM
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Re: Erratic idle bad WOT power

Originally Posted by Injuneer View Post

From my experience, spending the money for a cable is a lot less expensive, and a lot more productive than playing parts replacement roulette.
Amen Brother. I love the term "parts replacement roulette", perfection!

I too found out the hard way that even these "old" 93-97 cars are too complicated to just replace parts on a hunch or notion of causation. Not only do you risk spending money on replacing parts that don't fix the core problem but now have added a "new part" that may or may not be working properly. I work on electronics problems as part of my job and have installed circuits bought from the manufacturer that were not functioning right out of the box. When this happens it often sends you off track for quite a while because you just "know that part is working" because it came out of a box.

One other good reason to get the scan done first is that with each failed attempt to fix a problem you loose a little gumption, which is the will to fix the car. That is the key piece of insight I got out of Pirsigs' classic:

“Gumption is the psychic gasoline that keeps the whole thing going. If you haven’t got it there’s no way the motorcycle can possibly be fixed. But if you have got it and know how to keep it there’s absolutely no way in this whole world that motorcycle can keep from getting fixed. It’s bound to happen. Therefore the thing that must be monitored at all times and preserved before anything else is the gumption.”
― Robert M. Pirsig, Zen and the Art of Motorcycle Maintenance

Last edited by squarehead; 02-16-2019 at 12:01 PM. Reason: added thought
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Old 02-16-2019, 08:51 PM
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Re: Erratic idle bad WOT power

I’m working on getting a laptop available and loading the suggested software. Then I need to create or purchase a cable.

thanks for the direction.
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