95 GT | Unable to trace idle issue

[tpoteet]

Hi, I have a 1995 GT 5.0/AT 103k miles, entirely stock except the new 70mm throttlebody. I’m having an idle issue still after reading plenty of guides on the forum and doing some diagnostics. Basically, RPM drops to around 400 occasionally with smaller jumps between 600-800 with no regular pattern to it. I have noticed it is significantly worst when cold and smooths out slightly at operating temperature. I have checked voltages at TPS, 02 sensor, and IAC. I have replaced the following and associated gaskets: IAT, Throttlebody, IAC Gasket, Spark plugs(.054 Gap), Cap and Rotor set, EGR valve, PCV valve, vacuum hoses, and upper intake gasket. Picture down below for reference. Also have a video of data @ operating temp from a snap on scanner:

 

[tpoteet]

Also, IAT was plugged in for the data log. I know some may try to suspect that was the cause.

 

[ttocs]

What brand tb? I had all kinds of issues after I installed a 70 mm pro products tb.

 

[tpoteet]

What brand tb? I had all kinds of issues after I installed a 70 mm pro products tb.

It was a BBK, the issue was there prior to the TB swap when it still had the stock one.

 

[ttocs]

smoke check for vac leaks

 

[Michael Plummer]

FYI

Your camshaft profile will have the biggest effect on idle quality, while spark advance, A/F ratio, and your processor's ability to chase a desired idle speed thru spark advance will also play a role.

IAC: Plays a very small role in idle quality. Its main function is to allow air into the engine without passing through the throttle body. So it's more a function of holding/controlling your idle speed and NOT idle quality.

Throttle stop screw: Adjust ONLY on a HOT engine with IAC disconnected. Based on your combination of engine parts, you adjust to get idle that is smooth and consistent, based on your combination of engine parts, especially camshaft. On some cars that may be 650 to 750 RPMs providing you have a stock camshaft or 850 to 1000 RPMs if you have an aftermarket camshaft. If the car idles better at 900 to 950 RPMs then set it there. Don't set it at 650 or 700 RPMs because you think it sounds cool. Always remember this "Your camshaft profile will have the biggest effect on idle quality".

One quick tip on vacuum leaks: Vacuum leaks can be more than just the vacuum lines you can see. That's why you need a smoke test to determine if you have a vacuum leak. How is checking a vacuum line going to help you if you have a vacuum leak at an injector?

I hope this helps
Michael Plummer

 

[tpoteet]

FYI

Your camshaft profile will have the biggest effect on idle quality, while spark advance, A/F ratio, and your processor's ability to chase a desired idle speed thru spark advance will also play a role.

IAC: Plays a very small role in idle quality. Its main function is to allow air into the engine without passing through the throttle body. So it's more a function of holding/controlling your idle speed and NOT idle quality.

Throttle stop screw: Adjust ONLY on a HOT engine with IAC disconnected. Based on your combination of engine parts, you adjust to get idle that is smooth and consistent, based on your combination of engine parts, especially camshaft. On some cars that may be 650 to 750 RPMs providing you have a stock camshaft or 850 to 1000 RPMs if you have an aftermarket camshaft. If the car idles better at 900 to 950 RPMs then set it there. Don't set it at 650 or 700 RPMs because you think it sounds cool. Always remember this "Your camshaft profile will have the biggest effect on idle quality".

One quick tip on vacuum leaks: Vacuum leaks can be more than just the vacuum lines you can see. That's why you need a smoke test to determine if you have a vacuum leak. How is checking a vacuum line going to help you if you have a vacuum leak at an injector?

I hope this helps
Michael Plummer

It has been smoke tested twice, forgot I did not mention that in the initial post.

 

[ttocs]

the fact it changes with temp makes me wonder about the harnesses. I found my tps was showing 5-7% throttle at idle when I was not in the car. I swapped out the sensor and found the exact same problem but it was only happening when the motor was hot. I discovered the harness to the tps was bad and would loosen up with heat and the resistance it built would show up as a little throttle. I discovered the problem with the tps by lightly tapping on the top of it, and then diagnosed the harness problem but just grabbing the harness and pulling/tugging on it and watching the tps output. Try tapping on the sensors and then get all technical with it and jiggle all the harnesses while its running and see if anything changes.

 

[tpoteet]

the fact it changes with temp makes me wonder about the harnesses. I found my tps was showing 5-7% throttle at idle when I was not in the car. I swapped out the sensor and found the exact same problem but it was only happening when the motor was hot. I discovered the harness to the tps was bad and would loosen up with heat and the resistance it built would show up as a little throttle. I discovered the problem with the tps by lightly tapping on the top of it, and then diagnosed the harness problem but just grabbing the harness and pulling/tugging on it and watching the tps output. Try tapping on the sensors and then get all technical with it and jiggle all the harnesses while its running and see if anything changes.

I’ve done all sorts of things with the TPS, I don’t think it’s the TPS. 2 people who have driven in it think it’s a hard misfire at idle which is weird because when driven it is perfectly fine.

 

[ttocs]

I’ve done all sorts of things with the TPS, I don’t think it’s the TPS. 2 people who have driven in it think it’s a hard misfire at idle which is weird because when driven it is perfectly fine.

I was not saying your tps was the problem I was just using it as an example of how the harness can effect the sensor. Seriously give all the sensors some technical taps and then just mess/pull on the harnesses of any sensor you can find and see what happens.

 

[07GtS197]

Did you recently change or unhook the battery? Is it throwing any codes?

 

[tpoteet]

Did you recently change or unhook the battery? Is it throwing any codes?

Yes, it’s throwing some codes caused by a recent battery disconnect and reconnect and then a torque converter solenoid code.

 

[ttocs]

have you have the ECM out of the car or know if it has been out of the car?

 

[07GtS197]

I wonder if there were other codes. Was it acting like this before the battery was unhooked?

 

[tpoteet]

I wonder if there were other codes. Was it acting like this before the battery was unhooked?

Yes it was, let me try to check timing tomorrow and I’ll come back with the result. Seller I bought it from just gave me this response. Can’t even make this stuff up. Not to mention the timing pointer by the dampener looks like it got hit by a freight train.

 

[ttocs]

disconnecting the battery should not throw codes, or at least it didn't use to. I was asking about the ecm because what I read about the err says that there are two wires on the ecm that should always have 12v on them and to test them. IF the ecm has been out/replaced it could not have been put back together right as its a PIA to get it back up in the kick panel. Also just to be stupid have you checked all your fuses?

 

[tpoteet]

disconnecting the battery should not throw codes, or at least it didn't use to. I was asking about the ecm because what I read about the err says that there are two wires on the ecm that should always have 12v on them and to test them. IF the ecm has been out/replaced it could not have been put back together right as its a PIA to get it back up in the kick panel. Also just to be stupid have you checked all your fuses?

Have not checked fuses yet. To my knowledge it looks like it’s been there it’s whole life, but let me check timing tomorrow as the guy who sold me the car said he blindly set timing with a screwed up timing pointer. I’ll come back with results.

 

[07GtS197]

With a stock balancer you might need to verify tdc on piston one with a piston stop tool or screwdriver. But he probably advanced timing by turning the distributor. Erratic timing could also be an issue.

 

[Monday]

MAF??

 

[tpoteet]

So have timing set at 10 BTC now and it has smoothed out for the most part, minus the idle hang and occasional cam style idle, but it is a day/night difference that adjusting the timing made. Guy before me set it to almost 30 degrees BTC thinking he was advancing it.