I notice recently that the water temperature rises above target level once I start pushing it harder. The car is a 1991 6 speed manual l98.
On a racetrack it takes less that 2 minutes to reach the edge of the hashed sector on the coolant gauge. Not sure what temperature is there, 260F the end of the scale or the beginning of the danger zone?! anyway mine is a german edition and it has metric gauges and it’s 130C instead of 260F.
If the A/C is on it gets hot sooner and it would not go down even if I drive slower on a race track. Slower also means less cooling and still using some power.
If I drive normally on a highway or in city the needs stays in the middle.
The radiator looks ok from outside, clean. The coolant is 50:50 distilled water / antifreeze g11.
Weather these days was 86-90F with 83-86 at the racetrack.
When stationary the fans turn on and there is a hot flow of air you can feel if you place your hand behind the fans so it seems there is too much heat from the engine that heats the coolant. The pipe running in the radiator is so hot you can barely touch it (after driving harder)
and the one outside the radiator a bit lower l would say like 170F so it should mean the water pump is pushing the coolant well enough. I suppose stagnant water would mean cool radiator and hot in-pipe.
To me it seems the coolant system runs normally since I get the hot airflow with the fans running but is this behavior normal or a sign of a different problem like headgasket leak or EGR( Btw only on highway, not on track or city I get the h32 and SES) that heats the coolant much faster?
Last year I went on a track at 68F air temperature and I didn’t have water temp issues only the oil would go up in temperature after a few laps.

 

Do 91s have the LCD panel in the cluster like the later LT1 cars? If so - set to coolant temp. You’ll get an accurate digital reading. The analog gauge is more or less a guess.

Not saying you don’t have a problem, you may, but the analog gauge isnt giving you a good estimation of the actual temps.

Stock, these cars can run up to ~240F degrees and it’s considered normal. Too hot for many of our likings so we mod to run cooler.

Sounds like you covered a lot of the bases already. Is your air dam intact? It’s important for directing airflow over the rad.

 

I have the LCD but I don’t think it can display the temperature.
The air dam looks good.
Coolant leve in the tank is to the top and in the overflow tank is between cold and hot. If the water is hot the it seems to rise a little bit.
Also, the coolant temperature sensor is new.
I have a pressure pulse sensor (diy) and I can check the system to see if there is any pressure leak from the combustion but I need to find a adapter. Then I’ll check the EGR to see if it opens.

Here is a picture of a spark plug, they all looked almost the same.

I think it runs slightly leaner than it should, ?!

Thank you

 

That plug looks a little lean. If all of the plugs look the same, I'd suspect the fuel pressure regulator, fuel pump or an injector in that order.

 

I had a similar problem with my 1990 421. With a DeWitts radiator and pusher and puller fans it would run cool all winter but as soon as the ambient temperature went up the car would start running hotter (similar to your racing situation). I was ready to put the 350 back in it but got to thinking....If I brought it to a mechanic, what would they do? So, the first thing I did was take out the thermostat and test it. Darn if that wasn't the problem ~ it wouldn't open. I had drilled 4 holes around the perimeter of the thermostat body, and it would flow just enough coolant to let it run cool in the winter (in Florida). As soon as it got warmer out it wouldn't flow enough and the engine would run hot. Changed out a $10 part and not it runs 195 degrees in the hottest weather with the air on in traffic.
So.....check everything, starting with basics first.

Now, with the lean plugs. You had better pay attention with that (has it been tuned?) because you could lean out a cylinder pretty quick and lose an engine. If not tuned, it could be a sensor is reporting a rich A/F ratio and it's leaning it out a bit. Might be challenging with OBD1

 

It doesn’t overheat in traffic with A/C on and hot weather if I drive it normally, but if I give it full throttle on every corner exit on a curvy road the temperature starts to rise continuously. The air intake area for the radiator is very small, even if we add the section below the nose because air is pushed down to the intake, it’s smaller than of most of the cars on the road today regardless of the engine they have, but I can’t tell if that’s is the reason it’s overheating or not. If that was normal I could find ways to cool it down for example an extension of the dam to scoop more air from below or even a mist system using a high pressure water pump and a computer module.
About the A/F mixture, I’ve bought a new oxygen sensor that I will install soon, and I could also install a wide band O2 sensor to monitor the mixture. I’ll also check the fuel regulator, and see how I can test the MAP sensor. I can also pull a spark plug again because it’s possible also the car ran leaner before but not anymore. I understand the car relies on the O2 sensor to figure out the mixture and if it’s not within limits it would throw a code. However, I’m not sure if a bad O2 would be able to cycle the voltage between rich and lean correspondent values, It could be a little off I suppose.

about the dash, I’ve checked it again and it looks like my temperature gauge has a range from 40 to 130C and the warning marked zone should be at about 115C if the dial is linear. So if this reads correctly the water never went above the 115C -120C.

 

I could be incorrect as my car is a 94… but to the right of the cluster do you have a button that says “gauges”. That cycles through the diff info the lcd can display beneath your speed. Coolant is one of them. Check it out, it’ll give you a real reading. Hope the 91s have that!

 

These guys are right. Get that up so you can see what's really going on.

BUT: if the gauge is up to the hash marks, the car is running "too hot". jmgtp said that they can run up to 240....he's right that they CAN, but that doesn't make it "right". The car should run ~210...maybe 215 on a hot day at the track. Mine runs cooler than that; 195-210 ish? Depending on what I'm doing, on the track. At 200*, the IP coolant gauge should be pointed at about the middle of the range....not all the way up into the hash marks. So....he may not be "over" heating or damaging his engine, but we know that it's running hotter than it should be.

You either have an air flow problem through the radiator, not by design, but by debris between the condenser and the radiator, or you have a coolant flow problem (stat, pump) or you have a gummed up cooling system from lack of maintenance. I would remove the radiator shroud, clean out any/all debris between the condenser/radiator, then I would thoroughly flush the cooling system...and see how it responds to those efforts. On an L98, you could simply remove the T-stat to "Check" that; if it runs cooler, doesn't get to the same temps in spirited or track driving, there is your problem.

 

i track my 91 and 90. assuming u have the digital hvac control unit, do this,

the true digital readout can be found by pressing the up and down fan together on the hvac unit until 00 can be seen. then press the up arrow and scroll to 16. now press the middle button between the up down fan button. parameter 16 you just selected is engine coolant temp. that is digital temp that the ecu is seeing.

do this and let us know what it is. on track with extremely hard driving i get to 113c coolant. thats in a zr-1 which have overheat issues designed into them! your L98 should run cooler. i never see over 109c on the L98.

one thing, is your cats. if they are getting plugged, the engine temps, including oil temp will be higher and coolant temps will rise.

take it for that drive where u accelerate out of corners and obtain digital temp seen by ecu and report back.

 

Thank you

I don’t have the “gauges” button but I managed to get the digital reading by using the HVAC.
We’ll see what it says.
To me it seems there is too much heat into the cooling system. The t-stat is new and it’s 160F type, I’ve tested it before I installed it and I think it should be fine. Tomorrow I’ll check the coolant pressure.
It could be from the CAT’s also

 

Cats easy test: Remove oxygen sensor, then thread a compression test hose into the bung, run hose from that up to a gauge that you can view in the cabin...go drive it.

 

another way i was able to do it was with a laser temp gun. pointed before and after and the after was waaaaaaaay cooler than the before because no flow. i think i pulled up onto a sidewalk curb and laid in the curb ditch to get a clean temp shot

 

I have a thermal camera to check the exhaust but not sure how it should look …but I can post a picture
PS
I did some tests this morning but I didn’t have the camera with me, so I checked the cooling system with a pulse sensor. There were a few air bubbles for a minute after I pulled off the reservoir cap then nothing.

 

Should be hotter at the cat exit than at the entrance.

 

yes, but I think any catalytic converter has some degree of resistance that would cause a small or larger ΔT

 

Say what?

 

I thought it should be hotter at the entrance and if the Cat wasn’t working there would be a higher temperature difference . But I just saw you said hotter ar the exit under normal conditions. now it’s clear That makes sense since it’s supposed to burn the remaining fuel

 

here is a spark plug after doing 21 track laps and ~100miles highway/city


looks good I think but not sure how relevant it is

 

That looks lean to me. If you're running that lean under load, the engine temperature will increase significantly and probably overwhelm the cooling system.

 

Thank you.
I’ve checked the EGR and the solenoid stays open when the engine is shut down then it closes on idle so it seems that it’s working but the EGR valve doesn’t hold vacuum at all and it if I breathe in through the hose it smells like burned gas so it seems that it’s diaphragm failed?!
Anyway I plan to install an AFR to be able to check the mixture more precisely.