Excessive idle speed '92 PA

Hi Everyone,
I'm new here and I have a problem I am wrestling with. The other day my 1992 Park Avenue(not an Ultra) lost control of the idle speed. I was driving home from some errands and found my self having a hard time holding the car (http://www.buickforums.com/forums/showthread.php?t=5632#) stopped at a light. The engine stays at around 3000 rpm. By the time I got home the coolant was very hot, I suspect because I had to keep it stopped so much and the tranny fluid must have gotten very hot.

I did a scan and all the various sensors are giving readings in their respective normal ranges and there are no codes set. The fuel pressure is in range as well. I've been going thru the service manual and there are no procedures for trouble shooting this problem, only stalling, rough idle, surging etc.

Is there some kind of "Kick Down" from the tranny like in the earlier non computerized days that might be the fault?

Is there some master reset procedure for the computer?

Any ideas on what else to check?

Has anyone else experienced this kind of problem?

Any help would be greatly appreciated.

Thanks,

Mike

Did you test the TPS sensor readings through the complete range, from idle to WOT?

Thanks for the reply. Yes I have checked it as well as the IAC. Both devices are functioning normally. The funny thing is at idle the TPS is at .46Volts which is where it should be at a normal idle speed. I watched the IAC go thru the shutdown procedure when I turned off the engine and it performed normally. I removed the Throttle body and inspected everything that moves and found no problems.

My scanner (AutoXray) doesn't give me access to the fuel injector pulse width like it does for my Vette so I'm not sure if the injectors are putting out too much and the system is compensating by increasing the idle. The oxygen sensor is working as well.

Mike

Was the TPS reading (.45v) from the scanner, or did you check it at the TPS blue wire? The reason I ask is, they both need to be checked in order to verify that they have the same readings. This may be an ECM problem! I assume you have checked all vacuum fittings & lines for a possible leak.

BTW, there is no "master reset procedure" for the ECM other than disconnecting the battery negative cable for a few minutes. I don't think this will help with your situation. Also, the trans is electronic and does not have a kick-down cable.

Thanks for your reply, and yes I have checked the voltage at both locations. I am still looking for vacuum leaks. I did find something interesting today.

I pulled the plug from the MAS to check the voltages and ground. All is OK there and the car starts and runs very rough but not at 3000rpm. Now that by itself is not very interesting and the scanner shows an expected code 34 but it also shows airflow measurements based on my throttle position. It seems to track the normal readings ei. 4-7 gm/cm at low rpm's and around 20 at 3000rpm. I would have thought it would not change since there is no sensor generating the readings. Where do you think these readings are coming from? I plug the sensor back in and the code goes away and the idle speed returns to 3000rpm.

Thanks again,

Mike

When you disconnected the MAF and checked those readings, the ECM was more than likely in "open loop" mode. When this happens, the ECM will revert to preset values, often referred to as "limp home mode" or "fail safe mode" and will allow predetermined parameters to control engine function.

Basically, the engine will, (or should) operate without any sensor input while in “open loop” mode. Now, let’s look at it another way; if all conditions were normal and the engine was operating as intended and for some reason your MAF failed, what would happen. Your ECM would revert to “open loop” or, “limp home” mode control of engine and the MAF function, until the MAF failure was repaired. :screwy:

If all your sensor inputs and outputs are within specification and you have verified that there are no vacuum leaks, I would suspect the ECM has failed. BTW, have you used the “propane torch” (of course, not lit) to check for vacuum leaks? Also, you may want to use a vacuum gage on one of the manifold ports to test engine vacuum @ idle. I have four Buick’s and have had two of them develop this “high idle" syndrome. One was due to the TPS spring return being broken and not returning to the home position. The other was the PCM. When the PCM failed, I was able to see “TPS sensor low voltage” with the scanner.

When you disconnected the MAF and checked those readings, the ECM was more than likely in "open loop" mode. When this happens, the ECM will revert to preset values, often referred to as "limp home mode" or "fail safe mode" and will allow predetermined parameters to control engine function.

Basically, the engine will, (or should) operate without any sensor input while in “open loop” mode. Now, let’s look at it another way; if all conditions were normal and the engine was operating as intended and for some reason your MAF failed, what would happen. Your ECM would revert to “open loop” or, “limp home” mode control of engine and the MAF function, until the MAF failure was repaired. :screwy:

If all your sensor inputs and outputs are within specification and you have verified that there are no vacuum leaks, I would suspect the ECM has failed. BTW, have you used the “propane torch” (of course, not lit) to check for vacuum leaks? Also, you may want to use a vacuum gage on one of the manifold ports to test engine vacuum @ idle. I have four Buick’s and have had two of them develop this “high idle" syndrome. One was due to the TPS spring return being broken and not returning to the home position. The other was the PCM. When the PCM failed, I was able to see “TPS sensor low voltage” with the scanner.
Have you checked for a vacuum leak? Maybe a hose let go.

I'm back, thanks again for the suggestions.

I've been busy but I have had a chance to make some more checks. I have gone over all the vacuum lines and there are no leaks. I used gas from a tourch. I removed the IAC and cleaned the end as well as the bore it sits in. With the MAS disconnected the engine runs but want's to stall. When I plug the sensor in it immediately climbs up to 3000rpm. With it disconnected the scanner shows a Lean condition and the Oxygen sensor is around 44mV, kind of low. As I increase the rpm the Lean condition goes away and the oxygen sensor switches back and forth between lean and rich and the Cross counts increase. It appears to be working as designed. The system is in a closed loop condition. Fuel pressure looks normal.

I am at the point of spending money but I want to make the best educated guess I can. Each potential part is expensive and non returnable.

Does anybody with a scanner have the time and inclination to disconnect their MAS and see if the computer tracks the airflow as if the sensor was still connected like mine does? If it does I am going to buy the sensor if not I may opt instead for the Mem-Cal unit.

Thanks again for your help,

Mike

I'm back and a bit more discouraged. I took a chance and got a MAF sensor since it was the only sensor that when unplugged made a difference. Needless to say it didn't work, the very high idle still exists. I feel like I'm down to the Mem-Cal chip or the computer. I have located several units in junkyards but before I buy I was wondering about the VAT system. How would I reprogram the PCM too work with my ignition key? Do I need that Tech2 analyzer? I don't see any thing in the service manual yet but I'm gonna keep looking. Has anyone had experience swaping the PCM out?

Thanks again,

Mike

The VATS system, has nothing to do with engine control (ECU) operation. If you change the ECU, you will need to swap the prom from your existing unit to the replacement. You will notice a small cover on the back of the ECU with two small screws as retainers. Remove the screws and below the cover you will find the PROM. Slide the two latch retainers back and pull up on the PROM to remove. Replace in reverse. BTW, remember to always disconnect the negative battery cable, before performing this function! :iceslolan

Thanks for the reply. I have been going over the service manual and I see the VAT system has it's own module that tells the PCM that the correct key is inserted. On both my Vette's the Body Control module takes over that function. I jumped to a conclusion before I did the research. I ordered a PCM from a salvage yard that has the same Serv. No. in the hopes that I can swap the prom into my current computer. The thinking here is that the engine parameters are programmed in to the chip and if it is damaged then it might explain the excessive idle condition. The service manual refers to it as the Mem-Cal. It will be a few more days before it arrives.

Many others benefit from a thread, only when the person posts back and lets us know what happened.

Your, "It will be a few more days before it arrives" has turned into two months. Was it your PCM?

Many others benefit from a thread, only when the person posts back and lets us know what happened.

Chris, unfortunately this is the way a thread ends most of the time. Many folks come here for the sole purpose of finding a solution to a problem that either their mechanic or they themselves cannot resolve. When they finally find the solution, they are so relieved, they forget to come back and share the final details.
It is no longer an issue for them, so :gives:

I would assume that falcon9 found the problem was the ECU, otherwise the thread posting would have continued after a few days! BTW, the OP’s last post, and last activity in AF was; 9/28/07 @ 1:41 p.m. (The post above) :sorry: but, :wave:

Sorry Guys, I'm still around. The problem still exists and the car sits till I can get back to it. The computer swap didn't clear it. The replacement PCM wouldn't even let the car start so I swapped in my Prom and the car started but immediately went to the high idle condition. I am guessing it has some thing to do with the Prom. I do not have the ability to check its condition or program a blank one so I have to wait and find some one that can. I have been busy trouble shooting electrical gremlins in my 2000 Vette which is my daily driver. The PA is what I keep around for errands and winter driving. I have the last 2 weeks of the year off and I'll have to resolve it then.

Mike

OK then, problem solved. Here is what happened.

The other evening I went out to start the car and the battery was dead. My brother was outside working on his brother-in-laws truck, he has one of those Hemi Pickups and has been having trouble with the "Fly by Wire" throttle body. So he turned his car around and gave me a jump. While we were talking I noticed the entire exhaust system glowing cherry red from the Cat back to the manifolds. I had not seen that before because I had always looked at it in the daylight. We both immediately thought the Cat might be collapsed or clogged. The next day I put the car in the yard and got it lifted up and tried to separate the rear pipe from the Cat so I could look into the end and check it's condition. I could not separate the pipes so I took the entire system out from the manifold back. I started the engine to see what would happen and the fast idle condition remained so the Cat was not the problem after all.

So still confused but determined today I started to re-check every thing I could and still could not figure out why it was running so lean as to cause the manifolds to glow. The scanner still showed all the sensors working and the IAC (Idle air control motor) functioning. Since I don't have the ability to command it to change position with my scanner I was now looking for a way to see it actually move. I removed it and put a rubber plug in the hole it came from. I started the engine and all seemed normal once again.

In the end it turned out to be the IAC, I replaced it and the engine is back to normal once again.

Lessons learned? The scanner shows what the computer and sensors are doing but not necessarily what the moving parts are doing. The IAC was stuck open. I would never have guessed the small hole this device uses could pass that much air. Plugging it showed me where the extra air was coming from.

Now I am searching for a scanner or other device I can use for 2 way communication with the moving control systems on these engines.

So once again thanks for the suggestions and a Happy New Year to everyone.

Mike

Thanks Mike for the feed-back, and congratulations on finding the problem. When reading your first few post, you said; I watched the IAC go thru the shutdown procedure when I turned off the engine and it performed normally. I removed the Throttle body and inspected everything that moves and found no problems.I have checked it as well as the IAC. Both devices are functioning normally.
Based on your feed-back, we had to assume your scanner was reading IAC counts and the readings were within range. If your scanner does not have this capability, it would be a good idea to upgrade to one that can provide live data scan. The scanner market has become more competitive in the last few years and prices have dropped considerably. In addition, many used “full functional” scanners are listed on eBay.

My initial statements were true, I checked all the wires, voltages and lastly the resistance of the coils in the IAC. With out a scanner with 2 way communication I could only assume the counts my scanner was showing were being carried out by the IAC, a mistake on my part. Had I not decided to see if I could watch it move I would not have discovered the solution.

Mike