Elthesh

Ok, here's the problems. For quite some time now, the car has had stalling issues. If you keep the idle up a bit you can avoid stalling, around 1000 while driving, parked, at a light, whatever. Thats the earliest issue, but now I have a major new one. I was driving around today and got on an circular onramp. I don't know if thats relevant, but immediately after coming out onto the highway, rpms started dropping, car was stumbling badly, and any amount of throttle up to max did nada. After a bit of gradually slowing down to crawling speeds due to whatever was going wrong, I discovered that tapping the gas repeatedly occasionally got a quick surge in power which immediately went away, then I discovered that only lightly pressing the pedal made the car feel almost normal. I was no longer slowing down, it stayed on still with me keeping it at 1000 rpms about and I was able to finish driving home at a normal speed, even if I got up to that speed a bit slower than normal. Anything other than the very narrow range of lightly pressing the gas caused severe power loss, stumbling, everything. I did not notice any warning lights pop up either.

Does anyone know what might be causing this? At first I thought throttle body, then thought maybe a failing throttle position sensor and lastly I thought of the MAP sensor. This is only basic guessing from me though. The car did seem to go through a ridiculous amount of gas on the rest of the trip home like it was dumping fuel into the engine.

Things that have been done to the car in the past 5 years include new fuel pump, fuel level sensor, and fuel tank.

This is for a 1991 Trans AM, 5.0, TPI

blacksunshine'91

iTrader: (10)

When was the last time the fuel filter was changed?

Elthesh

I think it was changed when the fuel tank/pump/etc was replaced. I'm not 100% certain on that as I did hand the guy a bunch of parts and that may have been in there. That would be about 4-5 years ago.

Elthesh

Ok, on a hunch, I decided to test the TPS on the car. Now, IF I am testing it correctly, red lead on the middle wire, black to metal/ground, key on, the sensor seems to be reading horribly high. I can adjust it just fine, but the range shows up on the meter as a minimum of .75 and a max of .90. Seeing as its supposed to be set at .54, and I can't seem to get it anywhere lower than .75, and again, IF I am testing it right, this definitely seems to be a major issue. If these readings are correct, could they be the cause of my problems? I'm thinking yes, but would like to get some confirmation before I replace the thing.

This is just doing the adjustment with the bolts loose. Not manually moving the lever by hand.

Tuned Performance

iTrader: (92)

Tuned Performance

iTrader: (92)

Elthesh

Either my meter is screwed up or I'm doing something really wrong because I can't get a reading on any injector. I checked all 8, and while the numbers jump around a bit on first contact with both probes, they all settle on .00 although I think one was at .01. These are Ford SVO 19 pound injectors. At some time in the past I ohmed either the old Multecs or the Fords and got the correct .17 or thereabouts, but I don't know what I'm doing wrong or differently now. Meter on 20/200/2000 ... nothing.

Jp's camaro

I had a similar issue about a week ago, replaced the tps and its been fine since

Elthesh

I worked out the problem, finally. The IAC was either dirty or stuck. Not sure which or both, but I pulled it out, cleaned it up with brake cleaner and sprayed some in the throttle body. No more problems. After that, I pulled the pins out of the weather pack connector and replaced that piece as the factory one was crumbling to bits. So, dirty or stuck idle air control valve. Starts on the first try now.

KyleF

Must have been stuck open. Glad you solved your problem, but shouldn't use brake cleaner on throttle body parts. You should be using throttle body cleaner.

Drew

iTrader: (58)

And thanks for coming back to post the solution for future reference! Most people never come back to tell if a solution worked or not.


Myth. Or at least irrelevant around these parts. The origin is that back in the day some throttlebodies were coated with a coating that could be dissolved or damaged by carb cleaner. Now GM being GM, I've yet to see any kind of coating on a 80s-90s GM throttlebody. Walmart Supertec Carb Cleaner is just fine for our applications. You could use the throttlebody cleaner, but you're just spending more money to preserve a coating you won't find on a GM throttlebody.

Elthesh

I always try to keep posts of mine updated, especially if I fix a problem. The worst is when a post i find useful dies out with the OP never stating what, if anything, fixed the problem. For the cleaner, I didn't have many choices on hand anyway. I had brake cleaner, mass air and electrical parts cleaner. I didn't figure the last 2 would give me the clean I wanted and I did a basic search and got the standard 50 different answers and 15 different back and forth arguments, so I just used the brake cleaner. Thanks google! lol.

I still may change the IAC after I replace the gasket and give a more thorough cleaning to the passages in the throttle body. Since I know it was causing the worst of the problem, it may also be causing another related issue. Plus it hasn't been changed in 13 years anyway.

KyleF

There is a film on the throttle blades that can be damaged by other cleaners. Not sure what others would argue over.

Out of your choices, I would say brake cleaner would have been the best cleaner, just runs a risk of causing other problems.

Elthesh

If there's anything on the blades themselves, I didn't touch those. Just sprayed the pintle on the IAC and the passage where the iac plugs in. I think for larger openings like the main air intake on the TB I would hand wipe those down. Hmmm... That gets me wondering on buying a tiny flex brush like a straw brush to get in passages like the IAC one on the TB... Guess it depends on just how tiny the holes are. Possibly something like a gun brush or soft pipe cleaner...

Arguments I saw were basically people saying any of the myriad of cleaners out there are fine/safe/they've used it for years and others saying the opposite. Who knows, it's the internet.

Drew

iTrader: (58)

Um, yeah... No. There's no coating whatsoever on TPI throttle bodies that will be damaged by carb cleaner. Hell, the damn GM Service Manual says to clean the throttlebody with carburetor cleaner and a brush! Some people just don't know, what they don't know. Some of us have actually read the book written by the people that would know.




Under normal conditions, the throttle blades are supposed to remain a little bit open. That's what setting the "Minimum air" is adjusting. The IAC is a computer controlled leak, bypassing the throttle blades. Under most conditions, the IAC opens/closes the bypass to control the idle, specifically when coming to a stop light, when you let off the gas and the throttle snaps nearly closed, the IAC will open to keep the engine from stalling.

The minimum air setting relies on the throttle body bores and plates being clean. The idea of a mystical wizard coating keeping the TB plates from getting gunked up with carbon and other gunk, is nice, but it's wishful thinking. Because of the PCV and crankcase ventilation, EGR, etc you're going to have gunk in the intake. You need to get in there and scrub the back of the throttle blades and their bores to remove that gunk, or the IAC will have to do it's job and control minimum air. An old tooth brush and cheap carb cleaner makes short work of the greasy mess. I recommend going a step further, and hosing out the bypass passages in the TB, remove the IAC and clean THE TIP. Don't get solvent in the area between the pintle and the electrical body, don't try to pull the pintle or push the pintle, because that can damage the sensor. Just clean the gunk off the tip of the valve, and when everything is clean and dry, reassemble. Clean the bores of the TB, the edges and back of the throttle plates, just stay away from the shaft seals and anything electronic.

Personally I have cleaned half a dozen of my own thirdgens, a few more for friends and family, it nearly always improves driveability.

Elthesh

That methyl stuff listed in the manual would probably destroy more things than it ever cleaned on a car, lol. No wonder they list is as a do not use item. I'd hate to see the effects of someone pouring that stuff all over the engine. When I get in there to replace the iac gasket, I'm definitely giving the TB a good clean up. If that doesn't solve my idle issue, I'll replace the IAC and see if that changes anything. But a good cleaning is needed even though it looks reasonably fine in there. The IAC looked ok too, till I cleaned it and it got the car running. Looks can be deceiving.

Jorlain

iTrader: (4)

I think the throttle body coating is mainly a Ford thing... Or at least those are the only vehicles I've ever seen the warning stickers on. That being said, I haven't worked on a lot of imports, so take it as you will.

Elthesh

Seems I was a bit premature thinking I solved the problem. I had a chance to tear into the issue today and when I took off the plenum to check the EGR, there was nice fresh fuel in the plenum. I tested the fpr and some fuel came out of the vacuum port, so I took the entire thing apart and the diaphram was shot. This is most likely the source of all my problems. So, new regulator ordered and I'll install it tomorrow and see how it runs. I'm guessing it'll run much better at a minimum since it won't be sucking fuel into the plenum.

TTOP350

iTrader: (8)

Yup, that'll cause huge problems and the new one will fix it..

Elthesh

Alright, lots of progress. Installed the new fuel pressure regulator. First few starts, car stalled immediately which is to be expected, but it started immediately. No more slow cranking and cranking forever. It started to run better as it kept running but was hunting some and stalled once or twice more before running better and better. I let it warm up to get into closed loop mode and decided to drive it around the block. Drove it fairly lightly, but no obvious funny stuff happened and no more needing to two foot the brake and gas at the same time. I'm a bit out of touch about how it should feel when its perfect but it feels really good now. Parked it and turned the AC on which caused it to have a little random stumble, but I haven't really driven it so it may just need to burn the crap out before it runs really well, and with the AC on. An old vacuum hose rotted and fell off the T junction that is next to the washer tank and goes to the vacuum reservoir. Not sure if that would have any effect, but I will fix that next. Also need to check into the idle both in park and in drive to make sure its where it should be. Since its actually running and on its own, I can dig out the laptop and get some readings from the ecm. Once its cold again, I can see if it acts funny in open loop mode or if that was just due to running like crap for so long before the repair. Possibly may need to clean the IAC again, pull and check the plugs, etc. This is definitely major progress.