My 94 Coupe runs fine when started and runs nice and cool going down the road. The digital and analog temp guage reads about the same when running down the road.. between 190 - 200 degrees. But When stopped in traffic, the temperature goes up pretty fast on both guages, but they read a different temperature.. The analog guage is pegged at around 260 degrees, but the digital reads about 230 degrees.
Also when it heats up, the idle surges up and down and the engine runs very rough at lower rpms, but seems to smooth out when you accelerate more. As soon as the car starts going down the road, the temperature immediatly goes down back to normal.. 190 - 200 and the engine smoothes out.

I tried cleaning the IAC and throttle body, but that did not help at all. The fans do come on when the temp gets around 230 on the digital guage.. There is no check engine light and I am not sure if any codes are stored in the computer.

It seems as the idle surge has to do with the overheating.. I do hear fans coming on, but I am not sure if there are 1 or 2 fans mounted behind the radiator.. would 1 fan not comimg on cause overheating?

Where should I start with this issue?

 

The fans are programmed to come on at about 230 dg.It should start to cool off when both fans are on. In heavy traffic it is normal to get that high. When you put the a/c on both fans should turn on. I use my a/c in traffic to keep the temps lower. I dont remember the exact temps for the fans to come on but I'm sure someone will chime in. I always use the digital gauge-the analog is not accurate

 

Anyone else?

 

When the car isn't moving, there isn't any airflow across the radiator fins. So, the temp will climb until it reaches the "fan on" temp.
I think it varies a little depending on year, but it will be in the 225-230 range (looking at the digital gauge).

It's not that the analog gauge isn't very accurate, but the fact that it's measuring the temperature at a different location.
The gauge also isn't linear, so it's next to impossible to know what temp it's actually reading.

Look at your gauge again, and you will notice a shaded area at the upper end.
260 is the max reading on the gauge, and the gauge needle will just about get to the shaded area when the digital gauge reads in the 228 range.
Look close, and you'll probably notice the needle doesn't actually get into the shaded area.

Also, as enventr mentioned you can turn on the A/C to force the electric cooling fans to run. This is one car that will actually run cooler with the A/C on.

 

It does seem to not get past 230 or so on the digital guage, but it kind of worries me when I see the analog guage almost pegged...

Any ideas on the surging and running rough after the car is warmed up?
As said, I cleaned the IAC and the throttle body, but that didnt help.

There is no check engine light and I dont want to start throwing parts at it..

 

This is not totally correct for the fan operation of a 94. It fits the 95-96 years better. The fans are independent and do not come on together.


The primary (left) fan comes on (and stays on) with the A/C always. Or, if the A/C is not on, the fan will come on at 228*. If the temp gets down to 218* the fan will cycle off.
(Do this check, just look at it)

The secondary (right) fan comes on at 239*.

If the fans are not running at all the temperature will climb very quickly to the 228 mark. But when the primary fan comes on the temperature should not keep climbing especial this time of year when it is relatively cool outside (<80*).

If I get into traffic, I make sure the A/C is on and that way the primary fan runs and keeps the temperature usually under 215* but usually stays lower.

If you have both fans running at 239* it should definitely keep the temperatures lower than your indicated temps you have.

IF you fan operation is normal, than it is possible the thermostat is a little tired, or possible the flow in the radiator is restricted (lowering its efficiently with less air across it). It may need a cleaning or replacement. You should do a basic flush and change the coolant at least every 2 years.

**So far you have not verified anything for sure. If you have no fans running, dam sure it's going to overheat. But when running down the road you say it's OK so on the surface it sounds like the fans are not coming on.


I would get the temperature problem under control first and then see if the roughness problem is still there.

 

Your car is running normally as far as the temperature.
As said the 2 fans come on at different temps, 226 & 235 (on a 92) and probably what pc said for a 94.
Since it cools down when you get out of the traffic, it's OK.
If it's over 70* out, you can easily hit 207* sitting at 1 redlight.

As said, you can turn on the a/c. You can also install a manual switch easily (but I don't use mine until I hit 220* with more stop lights ahead).
As far as the analog gauge, best bet is to quit lookin' at it, not trying to be funny, most of those gauges do what your describing. I don't look at mine and I've checked the digtal, it is very accurate.

If your coolant's older than 2-3 years, the system needs it's regular maintainence - flush, including pulling the knock sensors.

You need to pull the codes before you even think about addressing the idle problem. Then follow the "rough idle/stumbles" chart in the FSM
IF you're going to keep the car, you need an FSM, you can't diagnois many problems on a C4 without one.

 

Both fans come on with the AC on my 94. Without the AC on one comes on at 228 and the other around 239

 

This is just hard believe. Your fan operation not of one of a 92-94 design or a 95-96 fan circuit design. It sounds like a cross between the two.

It has to be one or the other or something is not working correctly. The 94 uses one relay for each fan. The 95-96 uses 3 relays consisting of a series or parallel circuit configuration to run the fans in series or parallel with each other. They either run at half speed in the series configuration or full speed in the parallel configuration.

I know that models at the end of the year can have slight differences. But considering of what the PCM has to do to operate the fans, it's got to be one or the other. I don’t think there is a gray area here.

I can tell you what I described is how my 94 car works, as well as what is in the FSM as well as the Haynes manual also. As well as the consensus of many others.

If there is something else out there I would like to know.

 

I don't have a 94 FSM but on a 92, in the Cooling Fan Diagnosis, The second part of the Test Description says:

The cooling fans should come "ON" anytime A/C syatem is operating.

But, I've know the FSM to be wrong in a few places.

And, IIRC, I only get one fan with the A/C on but I'll verify this as soon as I get the oil pan gasket in and can crank it up.

 

have you looked up under the front and made sure you havent sucked anything onto the front of the radiator? I dont know how many times that has happened to me.

 

I didn't think it was right either. Could it be a bad relay?

 

Usually when a relay goes bad the contacts don’t make connection anymore and the device does not turn on. The coil in the relay can open up also making the relay defective as well. Also a relay can get stuck in the on or energized condition and then the device will be on at all times.

However in this case the secondary fan is does cycle on and off so it would appear the relay is working. But according to your description, it is doing it at the wrong times. Since the relay is being driven by the PCM it would appear the PCM would be at fault.

You can actually measure the low side of the relay COIL and see when the PCM pulls that point to ground. In the un-energized condition it will measure about 12 volts, and when energized will measure near 0 volts (somewhere less that 1.0 volt).

 

Please check to see if it says fans, or fan.

In my 94 FSM it does say fan…..no S, on my diagnostic flow chart.

 

I haven't had time to pull out the FSM yet but I seem to remember something in there about the AC pressure having an effect on the fans as well. If that is true I'm wondering if I have a bad pressure switch in the AC.

 

I believe your correct on this and might be couple more factors also.

Don’t want to hyjack this guys thread but some information I discovered relates. Hope he chimes back in with us.

For ever I have been preaching for the 92-94 models the cycle on and off temperatures of the fans. Well I broke out the FSMs to give a hard look at the diagnostic flow chart section. You can also find duplicated information in other areas of the books but so far I could only find it in that section. I also looked in the VIN J section as well as P. I thought all temps were 10* higher from the FSM, so I quote.

FSM
Primary (left) fan on…..216*, cycle off 208*
Secondary (right) fan on 228*, cycle off 217*

Haynes Manual:
Primary (left) fan on…..226*
Secondary (right) fan on 235*

I have been saying;
Primary (left) fan on…..228*, cycle off 218*
Secondary (right) fan on 238*, cycle off 229*

So I went out and did a test on my 94 vette. With no A/C on, the primary fan came on at 228* like clockwork. Did not come on at 219. So I am sticking with my story as to what I have been saying for years even though it is contrary to the FSM………
I did not let the sucker get up to 239* for the second fan, I was happy at this point.

 

engine coolant temp sensor! the one on the waterpump..............controls fan operation and engine timing based on how hot or cold it is............had a 93Z28 long time ago start acting up much lke yours sounds like, great cold but all stumbly and surgey when it got hot, i disconnected the ECTS when the car was running, the CE light came on, fans came on, but somehow the car ran better! went to the local dealer (jimball pontiac) and got a new ECTS.........cured the problem! .........thought id add.....

 

A definite reasonable suggestion. Probably not a bad idea to check the resistance of the CHT sensor using the temperature vs. resistance chart.

The programming in the PCM does cycle the fans on if the sensor goes out of resistive range. It might open or short also. But this would show up on the digital display as a wacko temperature which it does not. It is within range although accuracy is not verified. It would also probably throw code 14 or 15 with a MEL light.

 

Hi Im just a newbie mechanic but believe me I have had alot of problems with my 1994 vette just read my post under donkeydawn11



for the idle issue this is what fixed mine

check the voltage on the wires from the coil to the opti wiggle them at both ends the pig tail get loose at the plug going into the opti and sometimes needs to be replaced. 2nd check and recheck all spark plug wires

I replaced mine and not thinking I put the new plugs in and the coil plug which is hard to get on the top of the opti I used a pair of long nose pliers and it made a small cut in the plug wire that I couldnt see until one night I listened to the car and I could hear a low popping noise from under the hood I looked and seen a spark replaced the wries a second time and it was fixed.

 

With the a/c on, the PCM drives the fan(s) based on the signal from the Pressure Sensor. That's a thermistor like device meaning the resistance decreases as pressure increases. The PCM sends a 5 volt reference and each volt that's returned is approximately 100 psi. The target for most mobile a/c systems is to maintain the high side at around 200 psi (2 volts from the Sensor) which would give the liquid refrigerant a temp of about 100 degrees and it needs to do this whether the ambient air at the Condensor is 0 or 100. That's why the fans come on sooner (assuming a 50 degree or better air temp) and coolant temps are lower (assuming the a/c is working right) when the a/c is on - if the fans remained under control of the Coolant Temp Sensor - or Coolant Temp - refrigerant pressure would spiral out of control and either trip the High Pressure Limit or blow a hole in the hood.

I doubt any of this has anything to do with a poor running engine and I'd suspect the injectors (check resistance hot/cold) or ignition (Check Engine Light on?) or maybe even a cracked PCM board (aim a hair dryer at it when it's cold).