Good afternoon, all,

I have a 1986 L98 auto coupe that is still stock. I posted about this problem a while ago and thought I had resolved it; sadly, it has surfaced again. The problem is that when the coolant temperature is 200 degrees or above and I give it 50% or more throttle, the engine emits this awful screeching/scratching noise. With that, I notice a loss of power while the noise is emitted. If I let off the throttle, it goes away immediately, but returns again when I press the throttle past 50% or so. Alternatively, as soon as the coolant temp. drops below 200, it does not do it anymore. I worried it was a blown head gasket; however, my coolant levels haven't dropped, the coolant looks clean, the oil looks clean, the spark plugs are not fouled, and there is no smoke. I will look into a compression test next. Furthermore, the temp. never goes above 228 degrees (which it only gets to if idling for a long time in the summer; when moving, it rarely gets above 210).

I've replaced several worn out items, including the alternator, AIR pump, and tensioner. I have also replaced the O2 sensor, since it looked like it was still the factory one. I was told that it could be an air bubble in my cooling system that is causing the noise; the power loss if from the knock sensor picking up on the noise thinking its a knock and retarding the timing. I did have my coolant system serviced soon before this started to occur. Again, this is what someone suggested. I've done 2 cycles of burping (about to do a third) to see if it helps. I am seeing some air bubbles, but nothing too extreme.

If anyone has any thoughts, please let me know. I would really appreciate the input.

Thank you,
Nathan

 

Does it make the noise when you drive the car with the belt off completely? Does it make the noise at 50% throttle in neutral, or only in motion?

 

I haven't tried with the belt off, yet. And it only does it in gear, not in neutral. The noise sounds like its coming from the front of the car, though. Not from under or behind it.

 

Will it do the noise if you're not moving but in the right temp and rpm range? Same thing if you know about what rpm range it's happening in, if you sneak up to it on the highway, do you still hear it?

Don't discount things like slipping serpentine belt, bad idlers or tensioners. I even had one car that howled that it turned out to be the air filter not sealing right was the culprit.

 

Hmm, the details thus far makes me suspect it's the belt, or a pulley. I can get mine to chirp by punching the gas hard. Test it sans belt and let us know the result.

 

I have actually already replaced the idler, tensioner, and the belt for good measure (they were worn out, anyway).
It occurs in about the 2000-2500 rpm range and only under load, not in neutral. The sound is definitely coming from the front of the vehicle.

So, I just got back from driving and something new occurred. It did its usually screech once above 200 degrees. However, this time I did not let off the throttle; I stayed in it even through the power loss. It screeched for about 3 seconds then stopped, tripping the check engine light. After that, it would not do it again and power was back to normal (maybe even better than before). I romped on it and it made no unusual noises. Additionally, it no longer has any of those fuel smells I associate with an "older car." It runs fine until I turn it off and on again. Once I restart it, the "service engine soon" light is gone. If I accelerate hard above 200 degrees, it'll screech for about a second, go away, trip the light, and run perfectly after that.

For me, this seems to indicate it is a faulty sensor, something that ties in to the coolant system. Maybe the knock sensor, but I'm not sure.

Please let me know what you think and if my assumption is off base. Thank you all!

Nathan

 

Read the stored code and see what the car is trying to tell you. Screeching could be anything from the water pump to the torque converter. If you feel a loss of power, again, check the codes.

 

You would do that?

 

Yeah, that's a way to diagnose belt noises, no?

I didn't say he should go cross-country like that, but its a quick way to rule out the belt/pulleys.

 

So, I had the code finally read and it's a code 43, which has to do with spark. Any ideas?

 

I doubt the Code 43 is the cause of the screech. 43 can be set by the knock sensor, a bad knock sensor connection, a faulty o2 sensor, or more. I'd advise to wipe the code and see if it show back up soon. The knock sensor may pick up the screech as spark knock. How's the trans fluid look? It could be the TCC is slipping when under load because the fluid is bad.

Do you have a sound clip of the screech? It may help in diagnosis some.

 

I don't, but I'll see if I can get a sound clip and upload it. I'll wipe the fault and see if it occurs again. And the transmission fluid looks good, it doesn't smell burnt and still appears red.

 

Hows the fluid level read following the manual's instructions? Engine hot and running, on level ground, after rowing through the gears. If too high or low, the TCC could hiccup.

 

I actually have the original manual to the car, surprisingly, lol. I didn't check the level, just checked color and consistency. I'll check the level too to be sure.

 

Not unless you want to cook the engine

 

How quick does your car reach 200? You should be able to drive for at least 5 minuted without even needing to worry about it. Unless there is another easy way to check the belt and pulleys, trial by removal is my go-to. All OP would need is one half throttle pull with it off to verify the belt drive isn't making the noise, simple as that.

 

Driving for 5 minutes and a half throttle pull without a water pump could crack a head.

 

Doing a full throttle pull at any time can crack a head, or a piston, or spin a bearing, or drop a valve. One pull isn't going to increase the chances of a failure to unsafe levels. Guy I know drove a C3 50 miles with no water pump belt (belt was thrown), it was fine. There's Jeeps that drive around their impeller fins all broken off and face no issues. OP has already done it with the belt off, same as I did, and go figure, no cracked heads. His squeal persisted, I found my was due to the belt.

 

Except the OP says the noise happens over 200.
Hilarious BS

 

Since I'm not having other symptoms (no knocks, no detonations, etc.), I'm not sure if it's a faulty knock sensor. The code 43 may be a symptom, not necessarily the cause. That being stated, I do run 91 octane. I do this because I had some pinging (quiet, but there) a while ago when taking off from a stop (no codes).

I've read that C4s shouldn't need 91; could this be a contributing factor? Or could the 91 octane be covering up other symptoms? I've also read that a old/clogged fuel filter can cause ping due to lean mixture, resulting in a code 43.