i am at my ends here, i have a 90 L98, and i am having an intermiten problem, half the time the car just runs like a scared cat, and other times its wanting to die at every stop light, and when i go to put my foot in it, it stumbles really badly. i have run the codes before, and i got a rich condition... i have already changed out the coolant temp sensor, and the oxygen sensor, where should i look next? i also noticed the fan coming on waaaaaaaaaaaaaaaaaaaaaaay before it gets hot, earlier today i went to go meet up with my folks for dinner, and when i left about an hour later the fan came on when i started it. so what should my next search be?

thanks

 

There are 2 coolant sensors, one for the fan and one for the computer. Maybe replace the one you didn't. You might have a leaking injector or maybe a bad maf (I'm not sure if your year has one).

 

You need to observe the Coolant Temp Signal which is used for fueling. The colder it is, the more fuel it gets. The only way to do that is with a Scanner. Something appears to be up with the Sensor wiring or ECM (since you replaced the Sensor), but you won't know what to troubleshoot until you can see it.

 

The 90 model is speed density.

I am having a similar issue. If I am reading this right, when the engine is cold, the computer is sending more fuel into the system causing a stumble?

Mine is sputtering around when it is cold. The warmer the engine gets, the better it runs.

Not trying to steal the thread, but I htink we might be having similar issues.

 

mine runs great when its cold, but sometimes the warmer it gets, then it stumbles... where is this second coolant sensor located?

i also need to add that the cooling fans come on waaaaaaaaaaaaaaaaaaaaaaaay to early, i just started it up, turned on the defrost to warm it up faster, adn after about 5 mins of running the fans kicked on, and the temp gauge was still reading cold

 

On any car, there is only one Coolant Temp Sensor and that includes the L98. On the '90, it's threaded into the Manifold below the Throttle Body. In essence, it replaces the Choke. A thermistor, it decreases resistance to ground as it warms up. Connected to a 5 volt reference from the ECM, more of that reference reaches ground as it heats up. The ECM uses that voltage drop to calculate Coolant Temperature and with that, pulses/drives the injectors to deliver more or less fuel. When it's cold, more resistance to ground, the ECM senses more of the 5 volt reference and you get more fuel; hotter is the opposite.

The Sensor is also used for fan control when the a/c is off. For the '90 a temperature of 226 drives the Main Fan.

You need to troubleshoot this circuit because your fan is coming on when it shouldn't be; ie, the ECM is sensing 226 degrees and the motor is cold OR you have a current Trouble Code for something and the ECM has cranked on the Fan, OR you've got the a/c on which uses a different sensor to make sure the Fan is keeping the Liquid refrigerant in the Condensor from blowing a hole in something.

You also have a Rich condition - that would indicate that the ECM is sensing a temperature that's lower than what it actually is.

You view all of this with a Scanner - guessing at it is a waste of time - but any circuit that uses a resistance device to vary the input needs to be free of anything that would add or subtract to the signal - ie, poor connections and abraded wiring will easily muck it up, so pay particular attention to that.

Yes, there is another Sensor or Sender screwed into the right head between cylinders 6 & 8. That's used for the Temperature Gage in your Display and sometimes, because of it's different location, makes your Gage a couple degrees higher than what you see on a Scan. That doesn't matter - the ECM is only going use the Coolant Temp Signal Input - you gage isn't monitored.

 

i know on my 84 there was a sensor in the back of the AC compressor that worked the fan also, but on the 90 there is no sensor on the compressor


the check eng. light came on last night, but the code reader is up at my parents place, so i'm gonna snag it this weekend, and see what is pissing it off now

 

Check the grounds, esp the ones on the rear of the heads.

 

i'm not quite sure its the ECM, i poped the hood tonight to get the svc # off the ECM, and the unit on it looks brand new, and it has a remanufactured ECM, so someone has already changed that

 

ok, i have been doing some poking around now that i had time to look at it, it does have a newer ECM unit on it, thing looks like its probally a couple years old, i noticed there were a few wires that have been probed so there must have been an issue with it before i owned it, i had it running out in the driveway, and after a few mins i flipped on the A/C, and within a few seconds the primary fan kicked on, and according to this post this is a normal thing, would the TPS cause an issue, also where is the MAF on this car? the sensor between 6, and 8 is for the temp gauge

 

No MAF on a '90.

Probing sensor wires is a good way to introduce resistance or cause an intermittent short to ground. So are splices so make sure no one spliced in a new Connector. Try wriggling those wires around and see what happens.

I'm on my 5th or 6th ECM with 60K - hell, 3 were replaced under the original warranty, but the '89 had more ECM issues than the '90. When I lost my last one, the CTS signal was hopping around in the 200's, cold starts were impossible, and yes the fan was on at startup. I picked up on it with a Scan and then measured the reference hooked up and disconnected at the Connector and Ecm. Those were normal so I took the ECM to my Dealer and exchanged it. I have no idea what crapped out on the Board.

You could troubleshoot the TPS with an analog meter. As you open the throttle, the voltage should increase and be linear. If the needle jumps around, it would indicate a faulty TPS. What you've described doesn't fit that scenario as this wouldn't be intermittent - everytime you accelerated it would buck around.

Different years use different parts to signal that the compressor is engaged.

 

Newb at the Vettes but my 90 L98 Stumbles at idle and sometimes under light acceleration. The guy that I bought it from this week told me that he had narrowed it down to the EGR valve. Not sure if that could have anything to do with your issue but for what it's worth. I know that the T-stat housing is right around in there.

Jeff

 

i ahve been driving it around all day, and hasn't missed a beat, gonna take it out this weekend to see if it acts up, good thing about taking it on a corvette run tomorrow... tom wong will be in the group

 

Well I finally got it to throw another code, and it came back as an egr valve