The car is an 86 SR'ed 383, stock 48mm TB.

When the car is first fired up in the morning it idles nicely at 800-900 rpm - no miss, very stable. About the time I get to the first stop light, the idle has gone up to 1200-1300 rpm and will not drop. I ran a datalog on it during this and the IAC counts are 60ish at startup and 0 by the first light. So, it sounds like the ECM is doing its job.

I thought it might be a vacuum leak but wouldn't that show all the time - even at startup? What else should I be looking at?

Thanks!!

Dale

 

I can't quickly find in my FSM, whether 0 counts is fully open or fully closed. Based on your complaint, I'm guessing, fully closed. The changing counts for the IAC proves that the IAC is functioning, but the fact that it is pegged at it's limit, suggests there is air getting into the engine, unmetered. Several devices come on line as the engine warms up and goes into closed loop. The EGR is one that immediately come to mind.

Go with your first instinct and start looking for vacuum leaks... when warm.

RACE ON!!!

 

You're correct, 0 is supposed to be closed. I disconnected and plugged up all the vaccum fittings on the SR except the FPR and power booster (ie. no hvac/cruise/egr) with no effect. I also tried the "unlit propane torch" trick and did not see anything either. If there is any other ways to search for vacuum leaks, I am all ears.

It is interesting to note that if the a/c is on, the idle is normal (8-900). I don't know if this means anything or not.

Any ideas?

Thanks!

Dale

 

I agree with a vac leak somewhere. They are difficult to find usually, and there are many. You might hear a hissing noise to locate it. I tried the propane and not much success. So, the engine is running fine otherwise??

 

Engine runs just fine otherwise, smooth idle and accelleration, no missing, sluggishness etc. Just the high idle after it has warmed up.

I am not sure where else to check for leaks. I have unplugged everything but the power booster and FPR. The car pulls 19-20 in. hg while idling at 8-900 or at 1300. The only thing left would be the intake. I suppose it could be the SR but I am not sure how it could only be a problem when warmed up and not all the time.



Dale

 

Hi.
I have an 85 4+3. It has another ECM, but the principle is the same. If the IAC is closed there must come air from elsewear, which will say a vacuu, leak. Have you looked under the plenumto see if all these brittle hard vacuum tubes are all right. I had to change all mine due to breakage. Then follow the vacuum lines which are displayed in schematics on a label on the radiator scroud on the left side. Follow the lines and see if there are some cracks on the hoses. They are old so there is a chance that there is a leakage.
You should also check the canister vacuum valve. Check the vacuum tubes to the air systems valves.
Chech the runners for vacuu, leakage by blowing startgas or propane. If the rpm changes (increases) there is a vauum leak. i had new large tube runners installed and got a huge vacuum leak at the intake / runner part even though there was a new gasket . it was the machined part that was not even enough.
Good luck.

By the way have you done the minimum idle air adjustment? If there is false air coming in somewhere this adjustment is difficult to do the rpm will be higher than the 450.

Good luck
Jan-Erik

 

Since when is an 8-900 rpm idle, "normal"? As suggested I would go mack and make the minimum air and TPS adjustments, as per the book and see where you stand. It isn't easy trouble shooting a problem when it looks like part in the affected system is maladjusted.

I still think, that the problem doesn't occur until the engine is warm, is a major clue. Also, as mentioned, if the IAC has to close, doing all it can to lower the idle, and it still idles too fast...

RACE ON!!!

 

BTW, I also tried pinching off the line to the FPR and it did not help either.

For what it is worth, I dont think this can be caused by a leak in any of the attached systems - I removed them and plugged the intake ports with no effect. If I am wrong, please tell me.

If the leak is in the intake then why does it start OK (even when hot) then jump up within a few minutes (about 1 min when hot)? Shouldn't it happen all the time?

 

Well, the ECMs commanded idle is 850 so it appears to be on target for that.

I was going to swap IACs with my 90 and see if that makes a difference, I can do the minimum TPS/IAC adjustement then, too. I just gotta wait for it to cool down out there

 

It idles at 850-900 rpms before it is warmed up. But I doubt the ECM is commanding 1300 rpms, hot. If it is, why is the IAC closed?


Swapping IACs isn't going to prove anything. You have watched to counts change, so you know it is responding. Unless, of course, the pintle is broken off or missing.


You ask a lot of questions of those who aren't there but are trying to help. You are there. Keep trouble shooting. Regardless if what could, should, or can't happen, the IAC is closed. FACT. The engine idles too fast. FACT. It is getting air from somewhere. Your assignment, should you choose to accept it, is to find out from or through where. If caught or killed, the secretary will disavow any knowledge of your existence.

RACE ON!!!

 

I did the IAC/TPS reset procedure according to the Vader86 instructions. The TPS is fine at 0.55v. Unfortunately, the car will not idle above 550 now. The ECM "desired idle" is 850 and the IAC steps are 160.

I tried swapping IACs incase the 86's was broken but it had no effect.

 

I'm not sure what the "IAC/TPS reset procedure" is. Have you readjusted the minimum air setting? The TPS adjustment won't change if you don't touch the throttle stop screw. You have jumped from one extreme to the other. First the IAC was forced to full closed and the idle was still too fast, proving it was getting outside air from somewhere. Now the IAC is fully open (160 counts IS the max, isn't it?) and it can only get enough air to idle at 550 rpms, pretty well ELIMINATING the possibility of outside air. Something is going on here, that isn't coming through in the translation. Please elaborate.

RACE ON!!!

 

Insert ground into Terminal A and B of the ALDL, turn key on. Wait one minute and disconnect the IAC connector. Turn key off and remove ground. (Disconnect the EST wire... optional), start the engine. Wait until it warms up and block the wheels, set the emerg brake, have someone hold the brake, ect...if it's an auto, neutral for stick. Locate the minimal idle adjustment screw and reverse as nessecary until idle is around 400-500 rpm. After adjusting the butterflys use a voltmeter on the TPS and adjust (.55v is good). Engine should barely run. Once it's where you want it, turn the engine off, put in park if auto and reconnect the IAC and EST. Disconnect the battery to reset the code 42. Start the engine and observe.

 

And THIS is the time to reorient the IAC. Otherwise, just take the warmed up car for a ride, exceeding 40 mph, for it to self adjust. The TPS is adjusted most easily, with the engine off, ignition on.

RACE ON!!!

 

DON'T SHOOT THE MESSENGER!!
Agreed, however counts/steps on his should be 0-255. Counts being at 160 would reflect cold start based on temp (mine is around 40* for that no. of counts)that the ecm 'sees''.
I believe you should be somewhere around 40 (for the rpm given and being modded), which if he is at 160 @operating temp would mean adjusting first to lower the counts. Min. air screw will need to be turned c.clockwise closing the tb blade. Making small adjustments here is a good idea.
Aside from all this IAC adjustment, it still sounds like you have a leak somewhere, what I believe CFI is trying to convey to you.

 

Yeah. I didn't know how many counts were the max, that's why I asked. Also I didn't know if 0 counts was fully opened or fully closed. He told me closed. I have ZERO experience with a scanner. There is more going on here that being reported. First the IAC was forced to full closed and the idle was still too fast, proving it was getting outside air from somewhere. Now the IAC is fully open (160 counts IS the max, isn't it?) and it can only get enough air to idle at 550 rpms, pretty well ELIMINATING the possibility of outside air. Or maybe we're getting our chain yanked.

RACE ON!!!

 

No, I am not yanking anyone's chain - and I resent the implication that I am.

The procedure outlined by RRT Vette is basically the one I used. Ignition on, engine off - connect ALDL A+B for 30 sec, unplug IAC, ignition off. Start car and adjust idle stop screw so engine runs about 500 rpm +/-. Shut off engine, disconnect battery, reconnect IAC, reconnect battery, start car and enjoy.

I reported what TunerPro RT gave me for desired idle and IAC steps - take it or ignore it. I only put it out there as a diagnostic aide - I have no idea if it means anything or not. Can I test to see if the IAC is actually moving and doing what it is told?

All I know is that before I did the IAC reset outlined above, the car idled at 8-900 cold and 12-1300 warm. Now I can barely keep it running at 500RPM +/- (as seen on dash and TunerPro RT).

I am looking for ideas to help narrow things down.

Vacuum leaks were mentioned - how can I find them? I removed most of the vac. driven parts with no effect. I tried the propane trick with no effect (I have never seen that work, but I am young). What else is left?

Thanks!

 

I tried propane and WD-40 to find my vac leak. It didn't prove to find it by increasing idle. My problem was a up and down idle and when I came to a stop (warm) the idle seemed a bit high. My RPM guage doesn't work in my 86 and I would assume it was around 1200 rpm. I tried 4-5 times to look for the leak and nothing. I could hear a hissing sound everytime the idle would do down. I ended up finding a leak the size of a pencil eraser in the PVC pipe that runs to the intake manifold. A lot of investigating and it sounded as if it were coming from the EGR solenoid...but not. You can use a piece of garden hose or stick to pinpoint a little better. My idle IAC step count is 50 per TunerPro RT at a idle of 1000 rpm. I do agree that you have a vac leak...unmeasured air entering the intake.

 

Using TunerPro RT what is your MAF g/sec?

 

Regardless of your experience with a scanner, I agree with your deduction. Supposedly, IAC cts. can go to 255 (max. open), I have never seen the IAC opened like that on the scanner because my prom is programmed for 160 max. for cold start somewhere around 40* and colder (which will produce an idle of approx. 1200rpm). When turned off, IAC will go to that programmed number for re-start. However, as the other inputs (sensors cts.etc.) 'see' a warmer temperature the, ecm/prom will use the predetermined cts. for the temp. Today being around 82* outside my ''cold'' start showed around 80 cts.
Most (tuners)set up when the w.temps. of 160*(and above) to the same number of counts (40 seems to be a very common ct. number by tuners). Factory programmed, it will change the counts to temp. all the way to stock operating temp. 195*+. These are the things I have seen using my avalable scan devices (datamaster, autox, tuner rt).
Even as numbers are concerned the scanner is a reference, as you said, the number only reflects an area in which the ecm can be within range to adjust. So if he is at O.T. and 160 cts. with an idle of 550 something doesn't line up here. (that number being out of range for the ecm to comp. but would still try to kick up the idle).
Another consideration here would be to try and use/find another scanner. I have heard of some of them giving false readings.
Sometimes using the scanner allows something more tangable method to recreate similar/consistant results. Thats why I don't use the helms method. I will get it close adjust slightly watch scan for closed loop, cts. temp. etc.) turn key off re-start and that will rest IAC cts. (much faster for modded car that needs more playing)
As you mention there are other things, the process, closed loop but to what OT temp ?(as heated 02 will give closed loop) all imporatnt when doing adjustments. Vac leaks, (I have read soooo many posts on SR stuff that this would be a consideration).
Sorry for the long post