Strange Noise under acceleration in 5th

I'm moving down to Georgia in a month, so this past weekend I decided to drive 93 5 spd SC from Chicago to Woodstock GA to beat the snow and salt.

Somewhere before Nashville (about 450 miles into the trip), I noticed that when I accelerate in 5th gear, there was a strange noise coming from the engine compartment. It sounded similar to a lifter ticking, but had more of a rattly sound to it. It only did it 1/4 to 3/4 pedal under acceleration, and would disappear with WOT.

The first thing I immediately thought was head gaskets, so I found a parking lot and let it idle for about 1/2 hour, watching for steam or moisture out the tailpipes. There was a faint odor of coolant under the hood, but no leaks anywhere and nothing coming out the back. No dripping, no white smoke, no coolant odor at the pipes. Revving it caused the SC to whine, but no rattly noise.

Since I was by myself, I had no choice but to continue the last 250 miles to my destination. The problem didn't go away, but didn't get worse. I made it safely. The car still starts, idles, and drives fine, and it's not down on power at all. It just makes that noise in 5th. It doesn't do it in 1st-4th, though.

Coincidentally, about 100 miles after the noise appeared, my VMM oil change went right from OK to NOW, and completely skipped SOON.

The car is completely stock except for the air filter and IC tubing.

I was hoping maybe I just got some cheap lower octane gas that would cause this? Any ideas would be greatly appreciated, but I'm back in Chicago and the car is in a garage in GA, so I won't be able to check anything for a couple weeks.

Please put my mind at ease and tell me I didn't do fatal damage to the motor somehow

Thanks
Eric

you got it, sounds like you were getting audible detonation, i've heard some folks describe detonation as a lifter rattling or a valve tap

the noise is actually the piston rings being blown out, an engine detonates when the cylinder has more pressure than it can compress, so that excess pressure has to escape some how and the weakest link is the piston rings

audible detonation is much less damaging than inaudible detonation which leads to preignition and blown pistons

with the stock tune the 89-90 ecu can remove up to 8 degrees of timing with the knock sensor, so its kind of odd that you would get detonation there, however 3/4 throttle is the optimum range that detonation is most likely to occur

if you have a "tune" on the car then you may want to consider removing some timing in that range, if you don't then you may want to consider getting a chip to resolve your detonation woes

Was your coolant level normal when you arrived at your destination once the engine cooled down?

Is there any evidence of coolant in the oil?

Did the engine want to overheat?

If the answer is no to the above then I don't believe heat gaskets is your issue.

Noise only in one gear under certain load's leads me to believe you have something loose in the drivetrain such as bad engine mounts, loose IRS bushings, a loose driveshaft flange, faulty transmission mount, etc. you get the picture.

The coolant smell could be a weepy hose, etc.

I would start there if it were me.

decipha said:

you got it, sounds like you were getting audible detonation, i've heard some folks describe detonation as a lifter rattling or a valve tap

the noise is actually the piston rings being blown out, an engine detonates when the cylinder has more pressure than it can compress, so that excess pressure has to escape some how and the weakest link is the piston rings

audible detonation is much less damaging than inaudible detonation which leads to preignition and blown pistons

with the stock tune the 89-90 ecu can remove up to 8 degrees of timing with the knock sensor, so its kind of odd that you would get detonation there, however 3/4 throttle is the optimum range that detonation is most likely to occur

if you have a "tune" on the car then you may want to consider removing some timing in that range, if you don't then you may want to consider getting a chip to resolve your detonation woes

Click to expand...

Or what decipha indicates could be the issue. He's been under the hood of these cars for a while.

I live about 45 mins from Woodstock. Let me know via PM when you're back in town and I can give you a hand to check it out.

Thanks for the input guys. And Mike, thanks for the offer of help. I may take you up on that.

Other than the air filter and IC tubing, Sony head unit, and Taurus cupholder tray in the lower storage slot, the car is completely stock. It's a 1993, and the only tune on it would be the factory tune.

I didn't drain the oil when I got there, but I did check it via the dipstick. It looked as clean as it was before I left, and smelled like oil with no hint of antifreeze. The temp gauge usually sits on the N while cruising, and never went above half way while idling or city driving before the fan kicked on. The oil and antifreeze levels looked like they hadn't moved. I did notice slightly damp spots angled out under the pipe ends while idling and fan cycling, but there was no hint of antifreeze, so I assumed it was just the cats working properly.

The car does have a slight vibration that starts at 70mph and disappears at 85-90 mph. The (3?) motor mounts were replaced recently with SCP solid mounts by the previous owner. I had the car at my mechanic for a once over when I bought it, and everything looked good and tight in the driveline. I had assumed the vibration was just an out of balance rim, but I will double check the driveline when I go back down.

I was pretty sure I just got bad gas because I was getting low on fuel, and the only station I could find was Valero. I've never heard of them. I usually try to stick to Shell/BP/Citgo as much as possible.

I thought it might be detonation, but never hearing it before, and only doing it in one gear, I wasn't sure.

I'll check it when I go back down, and hopefully driving it 300 miles with possibly bad gas didn't tear up the motor. This car is minty mint with only 91k on it...

Eric

That is indeed a clear description of detonation. You're only hearing it in one gear (fifth), because you're at low rpm/high load. The first thing I'd recommend is to clean your MAF, especially since you're running an oiled air filter.