Engine grinding noise?? 1990 GMC 1/2 T 4.3L

[Jay]

Ok so I have a 1990 GMC c1500 2WD with the 4.3L (just so whoever is 100% aware of which exact truck I have). I’ve done a lot of work to it but so far all I’ve done to the engine itself is a compression test, spark plugs/wires, Dist. cap/rotor, slight timing adjustment (for it was off) along with changing fluids such as oil as well..

Compression test read 120..125psi evenly throughout ALL 6 cylinders. Open throttle gave the same reading as no throttle. I’m aware that this is decent for a motor with 327K on it…lol

Anyway…It seems that ever since then that there has been a small grinding noise coming from the engine. It is more active when I give it RPM (assuming I’m under the hood when testing this). I took a long screw driver and listened to the heads and various places of the motor and cannot pen point where the noise is coming from. Nothing seems to have a noise of anything banging around or grinding or anything. I’m beginning to suspect that maybe the new distributor rotor I put in may be what’s causing this? I do sort of remember being thrown off by bit being a little taller than the older one? Just wondering if anyone else has had similar issues?

Topic

[college man]

Remove the rotor and crank the engine to see if the noise goes away.
You may have some timing chain slack.

http://www.ericthecarguy.com/faq/solving-automotive-performance-issues?start=6#MechanicalIssues

[Jay]

Thank you. Im going to check that. This thing is running real smooth and nice other than that slight grinding here and there. It doesn’t seem to effect the performance its just that noise. Especially since it didn’t do it before the distributor rotor/cap replacement. Any other suggestions for what I should check while I’m working on it?

[Rob]

I have – alot – of experience with the GM 4.3L ignition setup, pre and post Vortec. Pre Vortec the cap was tall (similar to HEI distributor), post is flat.

Stupid things I’ve encountered that caused a grinding noise include:

Not removing the two screws for the rotor that are taped to the inside of the cap. Yeah, that happened.
There was a definite grinding noise after.

Grabbed a cap and rotor from the shelf (Vortec, – read flat cap) and installed. Grinding noise followed.
Turned out the rotor was actually for a 2.5L L4 from a mid 80s S10, ever so slightly longer. Suspiciously the apprentices S10 was purring like a kitten…

Had one come in with a grinding noise, high mileage truck. Bushings on the distributor shaft were worn so badly worn that the rotor got lost inside the cap and had to stop to ask for directions.

I suspect your issue may be the latter. Check for signs of metal transfer between the rotor and cap electrodes, see how much the rotor deflects from center pushing it around by hand with the cap off.

[Jay]

WOW!!! Thank you. That was helpful and went right there along with what I was wondering. I recall the old Distributor rotor even being slightly shorter than the new one I put on it. I didnt think too much of it though since it was the recommended one for my truck. Figured it was a modernized change or something. Im going to do a little experimenting with the distributor and let you know how that goes. Thank you for the advice!

[Author]

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