Hello everyone.

My 1986 Firebird seems to be idling high and does not slow down. It started suddenly one day, few seconds after staring the car; it snapped out of it a couple of times thereafter, but then became a permanent issue.

I am therefore asking you for guidance as to how approach the idling problem. I looked for vacuum leaks, but that did not seem to produce any results (possibly did not know fully how to do it). The engine seems to be idling constantly at around 1825-1875 RPM and nothing I tried to do seemed to help.

There are a couple of codes that have been present for a while now- namely, vehicle speed sensor, mass airflow sensor and coolant temperature sensor. Vehicle speed sensor code appears only after the engine is warm and only when the car is decelerating from highway speeds, and it did not seem to be an issue. Mass airflow sensor code has been appearing for a while now, but a new sensor did not eliminate the code, thus I put the old one back in. Lastly, coolant temperature sensor keeps coming on only with the code reader hooked up, and still keeps appearing even though I just replaced the actual sensor.

Please help.

Thanks!

 

Check the voltage reading at the TPS... With the scanner and the engine not running, look at the TPS%... Should be 0%. Use a DMM on the blue and black wires at the TPS to get the output voltage as close to .55VDC as possible.

As for the other sensors, check the wiring, the MAF relay (right next to the sensor probably bolted to the radiator support), the MAF fuse (the fuse holder bolted to the frame rail next to the air cleaner and charcoal canister with the red and black wires in it), and check the wiring harness 4-wire connection at the rear of the engine by the dizzy for the CTS (only the injectors run through the front, the CSI and CTS run through the rear connection).

Do you have the gauges cluster or the idiot light cluster? The VSS has 2 different systems depending on the cluster.

 

Thanks for the pointers.

Checked the TPS voltage through the computer - it read 0.54V, so that is probably not the issue...

I fiddled a bit with the relays - they looked dirty, and I got the engine running well for a bit, until I shut if off and the issue reappeared again. I'm not sure which relay I fiddled with, since there were two of them (one on top and one on the bottom). Now, the two codes are still present - MAF and Coolant temp - MAF with ignition on is giving me a reading ~700, with engine started its as high as 12,000. Also, coolant temp with engine on shows temps ~350, while with engine turned off it shows the actual temp. I'll try to go to the store tomorrow and see if I can get the relay and simply try to swap it and see what happens.

Lastly, in regards to VSS, I have the idiot light cluster (you know, when you have a sporty car, its very important to know how much fuel you've got left... as far as RPMs are concerned - not so much ). VSS appeared last september when I tried to fix hook up the cruise control and something shorted. However, it did not cause any issues since then, and the code is not even present currently.

 

Few updates. Went to the store, got a new MAF relay - did not do anything. Checked the fuse, it was burnt, but probably shorted by me when I was trying to clean up the connector with a screwdriver.

Anyway, the coolant temperature sensor code remains. The car was able to snap out of it for a moment today idling normally, and the service engine light came on and off. When looked through the computer, the temperature keeps switching between actual and ~380F. The sensor was replaced by me two days ago hoping this would fix the problem...

Also, while messing around I screwed something else up, since now the engine runs all shaky, barely accelerates, as if one of the cylinders is out. Also, when started now it simply starts, revs up, then down, then it dies. Although I think this is something new caused by my fiddling around.

 

Remove the connector from the CTS and check for 5VDC between the 2 wires. Also, check to make sure that the 4-wire connector at the back of the engine is connected tightly and that none of the wires are broken or chewed.

Check all of your engine grounds as well to make sure they are clean and tight, and the single ground between the 2 frame rail fuses.

 

I'll check the 5VDC - what reading should I expect?

Also, the 4 wire connector - is that the one right behind the throttle body, containing wires between from egr cold start sensor and coolant temp sensor, located right behind the throttle body, running right underneath it?

 

The 4-wire connector I mentioned is back by the distributor... The one in the front of the engine by the TB is ONLY for the fuel injectors. The one in the rear is for the CTS and the cold start system ONLY.

 

Finally I found time to work on my car again.

So, I checked the wires - they look clean. Checked to voltage at CTS - 4.9V, so I think that is okay, too. My friend noticed that the vacuum lines coming from the charcoal canister looked really bad and worn - in fact, they would collapse under vacuum with the engine on. Changed those, although the car still smells like gas (something I realized today - sometimes gas, sometimes like fireworks, believe it or not).

Anyway, the problem persists. However, I found a way to fool the computer and have the car idle well once in a while. I would start the car, then shut it off and restart it right back up, with the gas pedal pressed down and it would then idle fine until it is shut off again. Most of the time this works, but if I shut it off and start it again normally, it would idle high once again.

I'm seriously out of ideas where to look at now. Any pointers? Thanks!

 

Well, you said that the vac lines at the charcoal canister were junk... What about the rest? Replace them and see what happens. Also, check the resistance of the CTS when cold and disconnected from the harness.

 

The rest of the vac lines seem good - only the ones around the charcoal canister were junk, although its questionable whether they caused actual vacuum leaks.

I'll check the resistance of the CTS - however, it is new, and its doubtful that is the problem. - it would sometimes throw in a code, so it was the first thing I changed when the problem started. Since then, I also changed the MAF relay, and vac lines since the problem started.

What I dont understand is why sometimes the engine can be "fooled" to idle the way its supposed to.

Are there any other leads I could look at?

 

Are you still throwing a code for the MAF? If not, I would like to know what the ECM says the air flow rate is. If it's not one of the rubber lines, I'd check the sealing of the intake, valve covers, intake bellows, PCV valve and piping, the IAC and the ends of the black tube connecting it to the intake (also where the tubes join together at the PCV valve connection). Also, check to see if the EGR valve is sticking open as this will also cause a vacuum leak (and exhaust flow into the intake which kills the engine at idle).

 

Did you by any chance check to see if the IAC is working properly? A bad IAC would cause idling problems. Also touch all the vacuum lines. If black rubs off on your fingers, they are shot and need replaced. Check to see that the throttle blade is not sticking open or that the idle adjustment screw has not been moved. If the metal plug is still on the back of the screw, then its still factory set. Also if any of your gaskets have hardened and dried out, they could leak causing a vac leak. This will cause a racing idle. I saw that you are getting a MAF code. This could be your whole problem as the MAF has alot to do with idle speed. Start by replacing the coolant temp sensor as they are quite cheap. The vehicle speed sensor is important as well as this tells the ECM what speed your car is traveling at. Work on eliminating all your trouble codes. Then if the problem persists, check the IAC.

 

Okay, so I had some time to work on it again.

One discovery I made is that when looking through the computer, the coolant temperature stays at constant 113, sometimes it would cycle between 136 and 113. With engine off, the temperature jumps to either 384 or 392. In other words, the engine never knows its actual temperature and keeps running as if it is cold - high idle and always rich.

I put a new MAF today, the problem persisted, so I put the old one back. Also, I got yet another coolant temperature sensor, and replaced that just to get the same readings again.

Any ideas as to why the coolant temperature simply cannot be read? I am quite certain that this is the problem, as the engine assumes it is cold all the time.

Also, I will have a look at the IAC.

 

Try disconnecting the CTS at the front of the engine and the 4-wire connector at the back. Probe the CTS connector's terminals with a DMM to make sure there isn't any shorts. I'll have to look and find out what terminals at the ECM need to be probed along with the corresponding connections at that rear 4-wire connector to find out if there are any shorts there as well. It's almost as though there is either a bad connection with the CTS system or a broken/chewed wire somewhere along the line.

 

Thanks for the reply.

My thinking now is along those lines as well. I have a few questions as to how to approach it as my understanding of circuits is limited to one year of college physics.

I disconnect the two terminals - one in the front, and one in the back - check for resistance and expect 0 ohms? That means the wire is patent, right? How about if there is a short/ground?

Also, please have a look as to what wire to probe in the ECM. I tried looking there today and the intense amount of "nervous tissue" there is really intimidating, and clearly produced no results.

 

When you looked at the ECM wiring, did you make sure there isn't any loose terminals? I know I had a ground pop out one time that killed everything. Also, along the same lines, check the grounds at the back of the engine, should be at least 2 on the pass side and 2 on the driver's side. Make sure those are clean and tight.

As for the resistance, if you get 0 ohms during the read between the terminals at the CTS (one probe in the yellow and the other in the black to start), you have a short. You're looking for a reading that is basically "OL", or over the limit of what the meter can read (infinite, or open). You're not trying to read the resistance on the wires themselves first, but between them, to make sure there are no shorts. Once you make sure there are no shorts, you can then check the yellow and the black wires to make sure the wires don't have either a high resistance or infinite resistance.

Basically, if the ECM is reading off the scale (384 degrees) with the engine not running, that means that the CTS circuit has low resistance. Basically zero, actually (top of the chart has 212*F and reads 177 ohms). I would guess we're looking for a short somewhere. However, this would actually cause the engine to start lean and end up rich (less fuel when cold because ECM thinks the engine is hotter than it is, and more when hot due to the reverse).

I'd still look for a vac leak somewhere besides the hoses, as I pointed out above, but after the CTS problem gets nixed.