Spark Knock Caused by Cats?

[Frank.Rizzo]

Hello!

I just recently bought a 5-speed, 1994 Ford Mustang GT that I found in great condition. It has 120K miles and is 100% stock. The only problem I noticed was that it was pinging under acceleration when I test drove it. I thought that maybe the owner (an older woman) was using regular unleaded gas or it just needed a tune-up. Immediately after buying it, I took it to a local Mustang shop. They performed a tune up, replacing the spark plugs, spark plug wires, distributor cap and rotor with all Ford Racing parts.

Unfortunately, the car still pings, even though I use super-unleaded (91 Octane in Southern California) and have tried different gas stations. If it is cold, it won’t ping until about 3200 RPM’s. If it’s a warm day, or if the car has been in traffic, or if the A/C is on, it constantly pings starting at about 2400RPM, under acceleration.

The local mustang shop is saying my catalytic converters may be to blame because they are original and may be getting “plugged up” so they suggest getting an after-market exhaust with new catalytic converters. I plan to replace the exhaust in the future, but don’t want to do it before I have done some research and found the headers and exhaust set-up that I want. In the meantime, could the pinging be something else?

I saw the post about trying Autolite 24's gapped @ .060-.061 and I will check into that, but any other hints would be appreciated.

Thanks for the help!

 

[bgjohnson]

errr, maybe but I would look at your timing and fuel pressure and check sensors mostly, but if your cats are clogged it might cause some problems that could even hurt your engine

 

[mike keirstead]

Timing....for sure if it does it when its hot

 

[jrichker]

I'm in the same boat - replaced everything I could think of except the cats & mufflers and still pinging at anything over 2000 RPM in 4th gear. I have swapped the MAF, computer, TPS, ECT, ACT, and all 8 injectors. I even tried blocking off the EGR and got no change in the ping.

I also have low vacuum, only 15 " or so at idle with a stock cam. I have replaced all the intake gaskets twice, and even changed the manifold (that's the reason for the Explorer/GT40 manifold in my sig). The Chilton's shop manual I have said that the low vacuum when there aren't any vacuum leaks is a sign of a restricted exhaust system. The kicker is that it is a MAC cat H pipe with less than 50 K miles on it. The ping gets worse when I hit a bump, indicating something loose, maybe the converter core shifting and causing more restriction. It may be either the cat converter or one of the mufflers has a loose baffle that shifts and restrricts the exhaust flow.

 

[hllon4whls]

What about carbon buildup? I know in higher mileage cars this can be a problem.

GM sells some top engine cleaner that is supposed to be very good for doing this.

You pump it into a vac port while running, let it choke the motor then let it sit for about 2 hours. After that crank it up, clear the smoke and most of the carbon is supposed to be gone.

This is what I was told from a friend that works at a caddy dealer for 18 years. He says a particular caddy has a bad piston design that they have to do this often or replace the pistons.

 

[jrichker]

Don't know about Frank.Rizzo, but I pulled the heads on mine and cleaned them real good. They only had about 40 K miles on the rebuild when I pulled and cleaned them

I've played with the timing ranging from 8-14*, and haven't had any real effect. It even pings when the engine is stone cold!