Starting running rough / crappy today

[john42]

Drove the Cobra to work today and it started running rough. It idles just fine but it sounds like a cylinder or 2 isn’t firing or the timing is off. So far I’ve cleaned the MAF intake and the air filter. Also pulled the distributor cap and cleaned that. So far it’s made no difference. The exhaust note reminds me of old lawn mower…. It got a pufft pufft to it which is new. Tried to check the timing but omg I’m getting old and I’ll be damned if I can read the numbers. My next step will be getting the car up on jacks and giving the harmonic balancer a good cleaning so hopefully I can read it.

Engine: 1990 302, .030 over
Truck Flow heads, cam, intake.

Looking for suggestions as I run through things.


Recent charges are: new Factory Five exhaust headers and new side pipes. Oil change 2 weeks ago.

[GoDadGo]

I'd suggest starting with the basics.


Did you change the plugs?
Also, did you check your wires?
Could the O2 Sensor be the culprit?

Good Luck!

[john42]

I'd suggest starting with the basics.


Did you change the plugs?
Also, did you check your wires?
Could the O2 Sensor be the culprit?

Good Luck!

New plugs last summer
Wires are tight and good. Tested for spark on all 8 just fine
02 sensor- how would I know?

[Ford & Jeep Fan]

Get a squirt gun (or windex bottle) with water and spray the EXH Headers (or manifolds) while its running. You might see a specific pipe or two where the water acts different. Those COLD cylinders are the cylinders not firing like the others.

This takes a whole 10 seconds to do.

[Blitzboy54]

Are you running a Sniper? If so, Pull the air cleaner and turn the key. After the pump primes the injectors should click. Check all 4 barrels for fuel. If one is dry your problem is one the injectors stopped squirting. If this turns out to be the problem there are some easy checks. Might just be the injector plug. One thing at a time

[Joel Hauser]

Like godadgo said, I would take a closer look at the the spark plug wires and their metal terminals. My car has been misfiring for months, and I thought I check every possible culprit. Then this weekend I plugged in the OBD reader, and had a code that said there was a misfire on cyl 1. I checked the plug, and it was good; then I looked inside the wire end insulator, and saw that the connector/terminal was rusted and corroded. The wires were only a couple years old, with only a couple thousand miiles on them. I swapped out the one bad wire with an old one I had saved, and the misfire disappeared. Maybe you'll be so luck.

[john42]

Get a squirt gun (or windex bottle) with water and spray the EXH Headers (or manifolds) while its running. You might see a specific pipe or two where the water acts different. Those COLD cylinders are the cylinders not firing like the others.

This takes a whole 10 seconds to do.

This was the money shot! Drivers side rear cylinder not firing at all. Cold as ice. I pulled the spark plug and it is fowled badly with what appears to be oil. I put my Bore-scope down the spark-plug hole and had a peak. Cylinder appears pretty clean.... some black carbon buildup but a lot cleaner then what I've seen in youtube videos as "bad". Honestly it looked like the clean/good version of what I've seen. I use Royal Purple Max Fuel clean/stabilizer in the winter storage times. So, maybe that helped. Anyway I cleaned up the spark plug as best I could and stuck it back in and it fired right up, running on all eight, the proper sound is back and on a quick road test it felt back to normal. I'm going to get a new set of plugs and do all eight. It does seem however, that my 1990 5.0 that has a whole bunch of racing years on it may be showing it's age.
I have been wondering where my oil is going. It doesn't leak, my garage floor is clean. It burns about a quart in 7-800 hundred miles. At least that is what I seem to replace every so often. Seems it is time to start keeping a more diligent record.

[GoDadGo]

I've never tested an O2 sensor.
Most threw codes (All Were Production Cars) so I simply replaced the sensors.

Good Luck!

[42Bfast]

You might want to follow up on Go Dad’s suggestion on the O2 sensor as well.
My experience with a couple different engines when they started burning oil is that they become a bit more problematic downstream from the cylinder. That being the O2 sensors and ultimately the catalytic converter (if so equipped) will need more frequent servicing/replacement. Obviously the one on the side with the fouled plug is susceptible.

[Norm B]

When you have time do a compression and leak down check. Your oil usage could be either rings or badly worn valve guides. An engine can run a long time with bad rings but badly worn valve guides can lead to a catastrophic failure.

[john42]

When you have time do a compression and leak down check. Your oil usage could be either rings or badly worn valve guides. An engine can run a long time with bad rings but badly worn valve guides can lead to a catastrophic failure.

I shall. Also going to replace the O2 sensors.

Thanks for the help/ideas!

[Railroad]

Check your intake manifold bolts. They do not take much tq for installation and seem to loosen up as gaskets compress and heat cycles.
Good luck,