I have a problem with my 87 Camaro. The car was originally a stock Lg4 engine which I converted to TPI 2 years ago. Several small mods have been done to the engine since that day but still have the same deccelerating problem with the engine. I guess many of you would think of this as a engine technical problem but the engine has been fully overhauled and checked for vacuum leakages and bas sensors and there is no fault at the engine. The engine is not throwing any SES codes either.

Anyway the problem is decelerating after part throttle driving. Have anyone of you had that kind of problem before??
The engine starts and fires up good and drives good as well. The WOT works good and the air/fuel ratio looks good. I have a Lc1 kit so I can read the air/fuel ratings. I have also disabled the EGR system and the AIR system at the engine. I am familiar with burning my own chips, so I am currently using TunerPRO RT as the chip burning program.

Do you guys think this is some tuning problem actually???
To me it feels like the engine is getting to much air or fuel when driving so when you stop the car there is still some fuel/air to be used??

I can give a couple of examples. driving at part throttle in 50mph and keeping the car steady. Letting the foot of the pedal and the car wouldn't decelerate. The car actually stays in 50mph. (I don't have cruice controll btw)

Another example is when I drive for about 50mph and stop the car totally like when meeting a traffic light. The idle RPM is about 1000rpm in D. If I put the transmission in P the RPM jumps up to 1500-1600RPM and slowly goes back to normal operating RPM which is 900RPM in park and about 650RPM in drive.

Any ideas???

 

Sounds like the IAC is bad. Also check a data log and see what the ECM is commanding the IAC to do. It should be closing down on a throttle lift as the throttle follower decays out. If it does this and the engine isn't slowing then the IAC or IAC wiring/connector is likely bad.

RBob.

 

You might check and see if the ecm is getting correct speedo input and also check to see if converter lock up is not sticking..just some thoughts

 

Maybe a worn out throttlebody !

 

I think the IAC should be working properly because I am datalogging the car and the IAC is moving as it should if I look in tunerpro. It is changing the steps upwards and backwards.

I am watching the MPH gauge in TunerPRO when driving the car and the speedo in the dashboard shows the same speed.

What are you thinking about when you say worn out throttlebody??
Bad gaskets or dirt???...I have cleaned the throttle body and replaced all the gaskets.

 

I was thinking the housing is worn causing a slight bind.

 

Could be, could be.

I guess I will first have to start with a new IAC housing and a new IAC valve..Even though the IAC valve was replaced when I did the TPI swap.

 

the throttlebody shaft, not the iac. you can remove the billows and slowly open the throttle by hand and watch for sticking and binding.

 

The throttle blades move freely all the way after what I can see. It isn't more than just a week ago I had the TPI system disassembled for some inspetion and mounted it again.

The blades are opening and closeing as they should. If I pull the blades WOT and let go of the shaft again they jump back to idle position.

 

Meaningless. That is only the commanded IAC steps. If the IAC is bad (sticking) it is not the real/actual IAC steps.

There is no feedback of the actual IAC position to the ECM.

Also, when the shift selector is in the OD position there is no engine braking. The car will just roll on. And, most calibrations (all?) have the TCC unlock in decel. Which also affects engine braking.

RBob.

 

How long does the car stay at 50mph for? If the throttle closes it can't get enough air to keep going and if the TPS tells the ECM the throttle is closed it'll cut the fuel way down. Either way I don't see how you could stay at 50 for long. Sticky cable? Bad return spring?

 

Ok, so it could be something like a bad IAC valve then??..Since tunerpro only shows the commanded IAC step. Thanks for correcting me on that one.

I know that the IAC isn't old. only about 2 years old or something, but as far as I know this problem have happened to me since the TPI swap actually.

I guess i would have to try to replace the IAC valve and see what happens.The throttle body itself have been bead blasted and cleaned out very well so there shouldn't be any dirt that is clogging the IAC valve. Or the blades.

This happends also in OD and in D. I know about the TCC. I have done some adjustments there as well since the car no longer has 2.73 gears and I think the stock low speed for the TCC is a little bit low.

But anyway the engine shouldn't rev up to 1500-1600rpm and slowly drop down to 900rpm when stopping the car after driving in for example 50mph for 20minutes. :-)

 

I don't know excactly how long but a few seconds before it starts to slow down. Sometimes more like minutes. I have actually been cruising through small towns without keeping the foot on the pedal and the car keep going in 40 to 50mph depending on how fast I was going in the first place. :-)

 

To check if the IAC is causing issues: warm up the engine, when the idle is at the correct speed unplug the connector to the IAC. Then drive it. Note that it may stall when placed into gear, so expect that and compensate.

There are other areas that can affect the engine not wanting to slow down. Too much SA will do it.

RBob.

 

I will try that test today RBob. My car is almost always stalling when I place the transmission in to gear so mayby this has something to do with that as well???

I actually did the IAC test some months ago and I just feel that the engine sounds and responses better with the IAC fully extracted.
Just by using the throttle blades if you know what I mean.
The test were you warm up engine. Turn ignition off, Ground the diagnostic terminal and put ignition on. Wait for 30seconds and then disconnect the IAC connector. Shut off ignition, remove the ground to diagnostic terminal and start the engine. I guess that is the way of adjusting the idle speed as well :-)