I have an '86 305 TPI that won't stay running now that its cold. When the weather was better it had no problems, but now it starts and runs up to 1500 RPMs or so and then drops off completely and dies. Sometimes it will stay running for a bit longer but at very low RPMs and rough idle, I can go and manually open the throttle and hear the IAC motor running and adjusting as soon as I open the throttle even a very small amount, then the motor dies. It seems like the computer is incorrectly adjusting the IAC, like it thinks the motor is already warmed up or something.

I've read some other posts here where people had this problem, and some of the things that were suggested as a cause are: low fuel pressure, IAC, TPS, Coolant Temp Sensor, vacuum leak, etc. I am certain its not a fuel pressure problem because the fuel system primes properly and will start every time, it just dies off once the computer tries to adjust the idle. I also put in a new IAC motor this spring, its working and its clean. TPS is not new, but I followed the proper procedure to set the base idle and TPS to .54V, I also checked its voltage as throttle opens, voltage increases smoothly. I am not so certain there aren't any vacuum leaks, but I don't get an SES light so it can't be that bad if there is one. I also replaced the MAF this spring and I know its working as well.

One thing I did notice though is a single green wire hanging disconnected near the driver's side of the engine, and a sensor in the block under the exhaust header. It looks like the wire was supposed to be connected to that sensor but the spade terminal on the sensor is broke off. I have two coolant temp sensors at the front of the engine, one for cold-start injector and the other for the computer, I think? And my O2 sensor is in the exhaust, so what is this other sensor for? Could that be the cause of it not running? I plan to pull the sensor out and replace it anyway, but I'm wondering if it could be the culprit. I have WinALDL on my laptop but I haven't checked everything with that yet, I'd really just like to know what that extra sensor is for.

Thanks!

 

That may be the knock sensor but not sure about it being on the driver side.

 

Yeah, I was wondering about that too at first, but it looks like the knock sensor is on the passenger side a few inches below the exhaust header, so I don't know what else this mystery sensor could be. I think I'll just have to pull the sensor out (if I can get at it) and go from there. Assuming it was a coolant temp sensor, could it cause the engine to not stay running if the ECM wasn't getting a temp reading?

 

Is it in the head between #1 and #3 spark plugs if so that would be a temp sensor. I'm sure it could cause some problems not getting a reading especially if it is the one that tells the IAC when the engine is warm and lower the idle.

 

I'd have to check to be sure, but I believe that's where it was, its tucked up really close to the exhaust. I thought the coolant temp sensor on the front of the engine is what the ECM uses. I have two sensors on the front of the engine, one is used for the cold-start injector and the other, AFAIK, goes back to the ECM. I actually replaced the cold-start injector sensor a while back and pulled the other one out at the same time, there is coolant behind both. Does the ECM use two temp sensors to control and monitor things? It does seem to make sense that a temp sensor is the problem though, because like I said, it seems like the IAC was adjusting itself if I opened the throttle even a very small amount, which then killed the engine.

Thanks!

 

One goes to the ECM (2-wire connector). The other goes to your temp gauge (1-wire connector). Does your temp gauge work?

It may be for your oil pressure sensor. If the oil pressure sensor is not functioning then you may see something like what you are seeing. The ECM will operate the fuel pump for a bit when you first start the engine. After that it relies on the oil pressure sensor to keep the fuel going.

I think that there are more then just one wire going to these though. Something to check anyway. I would also actually check the fuel pressure. Just because you hear it prime doesn't mean that there is enough pressure.

 

Ahh, that makes sense, I didn't even think of that. The temp sensor at the front of the engine is the two-wire one which I figured goes to the ECM, my temp gauge hasn't worked for most of the summer but I thought it was supposed to get a reading from the ECM, not from another temp sensor. Ok, so that's why my temp gauge isn't working, but I guess that won't help the fact that it doesn't stay running. I didn't think of checking the oil pressure switch though, I'll check that and the fuel pump relay. I remember the last few times I drove it before this happened I noticed that the fuel pump would run for a really long time when it primed the system with the ignition on, instead of 2 sec. it was more like 30 sec. or more. Fuel pressure regulator maybe? I'll try to check all this stuff out this weekend and post back here with the results, I may need more suggestions if none of that works.

Thanks

 

Might be the fuel pump relay if you were having issues with it running too long during the prime. Maybe it was getting stuck. I have never had an issue with the fuel pump relay on either of my Camaros but I think I have heard that it is located near the wiper motor on the firewall. Maybe more toward the fender on the driver's side.

 

It is the temperature sensor if it is a single green wire and on the drivers side between cylinders 1 and 3. Probably broken during header installation. Definitely replace it and see what happens.

 

that wouldn't have anything to do with the way the car is running. The only thing that sensor does is send the signal to the temperature gauge on the dash. It is not tied into the ECM at all. The 2-wire one in the intake is the one that the ECM uses.

 

It sounds like I'm gonna have to check the fuel system then, would I be able to properly check the pressure by putting a gauge on the valve on the fuel rail? Also, I'll need to get a fuel pressure gauge, is that something I could get from Autozone or Advance Auto Parts, or would I need to look elsewhere?

 

That's where it hooks up, and you can get them at most any parts store.

 

I was going to get a fuel pressure gauge but the cheapest one was $40 and I didn't feel like spending that money just to find out fuel wasn't the problem; I thought I'd do some other diagnostics first. So I checked the oil pressure switch, fuel pump relay, etc. all looked good. On a whim, I thought I'd check the MAF and air intake to make sure it wasn't plugged or anything, it looked ok, but I unplugged the MAF for the heck of it and tried starting it up, it ran like a champ with the MAF unplugged!! Now I'm confused, there were no error codes with the MAF plugged in, should the ECM store an error if the MAF was bad? I don't know if it means anything, but I tried pulling a vacuum line off while the engine was running with the MAF disconnected and it didn't really make any difference. I don't have a vacuum gauge but it felt like there was good vacuum in the line so could the MAF really be bad and not throw an error, or is there anything else I could check at this point? If I have to replace the MAF its no big deal, I got the one from Advance Auto Parts with a lifetime warranty, if it is bad they will replace it, I'll just have a hard time explaining to them that there are no error codes indicating that.

 

Not 100% sure how to test it as neither of mine have MAFs. I am pretty sure that you will only get a code due to a bad MAF if the ECM gets too weak or too strong of a voltage signal. Not if it is incorrect. Also, I think that by unplugging the MAF it may go into "limp" mode and run on a pre-programed setting instead of using all the sensors. I could be wrong on that though.

 

I'll see if I can test the MAF signal as well as the MAF power and burn-off relays. That does sound correct, it probably won't throw an error code if the signal is still close enough to normal, or maybe it just didn't stay running long enough with the MAF plugged in. You are correct that it goes into "limp" or open loop mode, but does the ECM stop relying on any sensors other than the MAF in open loop mode? Like I said, the IAC is new and the TPS voltage seems correct. One thing I did notice though is that with the engine running, the TPS voltage at idle is not steady at .54 volts, it jumps around from approx. .51 to .60, is that normal? The TPS does have a steady and smooth voltage increase as I open the throttle with the engine off.

 

I had a TPS that did kinda the same thing. The voltage would jump around and I had a bad miss at idle. I was messing with the TPS (Not adjustable on my car) testing voltage and resistance. All looked fine. Plugged it back in and the reading was still jumping around. After looking at injectors and stuff it just started running fine again. Checked the TPS reading and it was back to normal. Been running fine ever since. Still a mystery! But mine does not jump around anymore.

I believe that in open loop mode the only sensor that it ignores is the O2 sensor. I'm thinking that by unplugging the MAF it is forcing the computer to look at pre-programmed fuel maps and that is why it runs well.

 

So maybe the TPS jumping around at idle isn't critical, I suppose I could pull it out and clean it anyway, I don't think its ever been replaced.

I did also replace the O2 sensor about a year ago, but I'll check it as well and make sure its giving the ECM a proper reading, do you know off-hand what reading the O2 sensor should have? It sounds like it could only be one of a few things then: O2 sensor, MAF, MAF power relay, or MAF burn-off relay.

I'll post back here after I check those things. Thanks!

 

You can borrow a FP gauge from most of the autoparts stores for free. An EGR valve that's stuck open will do exactly what you describe.

 

I checked with every auto parts store in town, nobody has a fuel pressure gauge to rent/borrow. I hadn't thought of checking the EGR valve, is there any way I can tell if it's stuck? And if I need to replace it, does that involve removing the intake plenum?

Thanks

 

i have been having a similar problem with my 87 iroc 5.0 tpi for some time now.. is there a way to test the many different sensors on this car (oil preesure, coolant temp, etc.)?? or do i have to start throwing parts at it?? i hate to do that but i guess i will if that is what it is going to take to get this thing running.

 

From what I understand you should be able to test most of the sensors to some extent. For example, I believe the oil pressure switch that acts as a fuel pump cut-off should have continuity across its two terminals, if not, the 12v power doesn't get to the fuel pump. For the coolant temp sensor, it should be giving a voltage reading depending on temp, but it could be only slightly off at any given temp and still cause a problem, I don't know exactly what the reading should be though. Other sensors can be be tested as well, there are some good sections in the Tech Articles area of this site for TPS and IAC, etc. Also, if you haven't checked out WinALDL yet, I would suggest googling it and get a copy, you can also make your own data cable and get some readings if you have a laptop, this will help immensely when diagnosing the system. In my particular situation, I'm leaning towards the EGR solenoid or the EGR valve itself going bad, but I still haven't found the time to check either.

 

Great news!! I finally found some time to take a look at this again, and eliminating the EGR solenoid as a possiblilty, I checked the MAF itself. Right away I noticed the MAF wasn't getting any voltage, so I checked the relays and found a loose ground wire on the MAF power relay, the wire had actually pulled out of the connector inside the relay plug and was just sitting there, it could have been making contact intermittently but must have come loose just by driving around. I cleaned up the wire end and re-crimped it to the connector. I then plugged the MAF back in and it started right up, runs as good as ever now!