I recently bought a non running 95 Z28 A4 Convertible, 108,000 miles. I replaced the OPTI and got it running. The car seems to idle fine and ran ok (but not great) at higher RPMs when I initially took it for a test drive. After it fully warmed up it started "surging" so I took it back to my shop. I hooked up a fuel pressure guage and restarted it. Fuel pressure was good at idle. I tried revving it up to see how that would affect the fuel pressure. When I opened up the throttle it made a LOUD squealing noise that sounded like it was coming from the top/rear of the engine. It sounds a lot like a squealing belt but I am certain it is not the belt making the noise.

What is there at the top/rear of the engine that could be making this noise? I have worked on a lot of older engines but the LT1 and Fuel Injection is new to me. Could the injectors make a noise like this? Based upon the way the engine was running I thought it could be a vacuum leak but I have never heard a vacuum leak that was anywhere near this loud before. Could it be a vacuum leak?

Thanks, Joel

PS. I don't (yet) have a scan tool.

 

No, I don't think injectors can make that noise, all they ever really do is tick- if you can hear it over all the other noise.

It sounds like you need a stethoscope. I just got one from advance auto and it's helpful in diagnosing strange noises. I started listening to my engine and I'm thinking my wonderful squeaking noise I've had for over a month is probably my tensioner, not my alternator, as I initially thought. I know that the advance auto here is very helpful, too. Even with the stethoscope I was still having problems, so I told one of the guys there and he sprayed some wd-40 on my tensioner pulley and Mr. Squeaks all but went away. - Noise travels very strangely in an engine compartment, you can listen all you want, but you still can't localize the sound, it just sounds like it's coming from everywhere.

But the only things I know of that squeak are the belt and pulley assemblies up at the front- they start doing it when they get bad, and will be quiet when they're cold, and as they heat up and expand, they'll get louder.

Hope this helps. If it is something from the back, I'd suspect it's something more serious than a pulley.

 

Also- If you could record the noise and post it on here, it would be a lot more helpful, since a lot of different components make squeal-like noises, and it's hard to decipher without listening...

Most of the people on this board have had one form or another of annoying miscellaneous noises from their cars. - Just do a search for "squeal", "squealing", "squeak", etc.

 

I have not heard it with my own ear, but have read that others say the intake manifold leak might cause a squeal at the back. Just a thought.

 

I tried the stethoscope method to hunt down an engine squeak and convinced myself that it was the tensioner. After replacing the tensioner with no reduction in noise, I sprayed some belt dressing on the belt and the squeak was gone. Belt dressing is really only a diagnostic tool, as the squeak will come back in a few days. I replaced the belt and the noise was gone for good.

 

I know it can be hard to pinpoint the source of a noise but as I mentioned in my original post, I am CERTAIN that the noise is not coming from the belt(brand new). I am also certain it is not coming from the front of the motor.

Has anyone heard this type of noise from a vacuum leak? Is there any component at the top/rear of the motor that could make this type of noise?

Thanks, Joel

 

Yep, shoe box nails the obvious for us again, (why cant WE ever see this stuff, lol) mine whisled from the vacuume leak on the intake. Hope this helps, prorac1

 

what about a vacuum leak at the EGR valve?

 

It could be possible, though I have not heard of anyone complain of that with an EGR problem. Easy enough to disconnect the vacuum to the EGR to see.

 

I took the intake off and found no obvious leaks. Replaced with new gaskets and seals. No change. Still makes the noise when I rev it up and it still does not run properly when driving down the road. Will surge contiunuously, sort of dies then the power comes back. It seems to idle just fine. At idle it is very smooth. No sign of the noise at all. Are there any ported vacuum sources? If so, that might explain why it does not do it at idle.

I had some other people listen to it (I don't have great hearing) They both said it sounds very similar to a slipping belt but agreed the noise was definately not coming from the belt. Also said it sounded like it was sucking air but could not tell where from.

Any suggestions on what to check next?

 

I work at a carb shop, heres a good trick, run propane around with one of those little green propane bottles, if the rpm's raise at a certain spot on the engine, theres your vaccum leak, keep in mind if you run it infront of the throttle body, thats not a vacuum leak

 

I now have the car running well but I still hear the noise on occasion. The things that were causing it to not run right were the converter and MAP sensor. Converter was plugged and the guts were loose so I removed the guts completely. The map sensor had carbon built up in the port that sticks down into the intake. After I fixed these problems it seems to be running perfectly, or at least as well as it should for 108K.

Prior to making these repairs it would make the "squeal noise" any time I would give it any throttle. Now it only occasionally makes the noise.

Any chance that an exhaust leak would make this type of noise. Any time I have heard an exhaust leak (at least what I thought was an exhaust leak noise) it was either a ticking noise or a low tone. This noise is much higher piched. I am thinking that the plugged converter would have made it make the noise much more/louder (and it was more/louder before I gutted the cat) because of the greatly increased backpressure. So far I and several other people have not been able to isolate where the noise is coming from. Any suggestions on how to isolate/troubleshoot?

How does the "air pump" work? Could it be the source?

PS. It makes the noise with the belt off so it is definately not the belt or accessories, run by the belt that is making the noise.

 

If you have arching plug wires or any other source of spark, wouldnt you risk blowing yourself up?

 

no, propane might egnite and make a little puff but the fan of the engine blows it all away anyways, and if it does spark it wont get sucked into the propane canister it should jsut stay a flame at the end, propane is heavier than air and it sinks, so its good for finding intake manifold leaks. amke sure you do this in a well vetned area! and dont hold me responcible for anything bad that happens!

-Mark

 

well thanks for the info.