nasty misfires

[slobound76]

have a 77 280z and was cruising down the highway and had to get on the brakes pretty hard due to incredibly slow car on the offramp, she was purring like a kitten until the slowdown then started running horrible, would barely remain running. while running there was a real strong fuel odor and black smoke coming from the tail pipe. got her home and pulled the spark plugs and one was real black and there was oil in the plug hole.

thinking blown piston rings maybe?

anyone had this problem? thoughts, ideas?

[SteveJ]

have a 77 280z and was cruising down the highway and had to get on the brakes pretty hard due to incredibly slow car on the offramp, she was purring like a kitten until the slowdown then started running horrible, would barely remain running. while running there was a real strong fuel odor and black smoke coming from the tail pipe. got her home and pulled the spark plugs and one was real black and there was oil in the plug hole.

thinking blown piston rings maybe?

anyone had this problem? thoughts, ideas?

Okay, how about some details?

1. Did you slam on your brakes, downshift, both, or do something else?

2. Which cylinder had the fouled plug?

3. Have you done a compression test? What were the results?

4. Have you tried a fresh plug? Did it foul?

5. Did anything shift when you stopped hard (such as the battery)?

[slobound76]

braked pretty hard, downshifted to 3rd, was 5th cylinder, don't have facilities/ equipment to do a compression test at home, replaced all plugs and no difference, and battery actually shifts every time i drive it, on my list of things to do (just bought the z as a project car)

[SteveJ]

braked pretty hard, downshifted to 3rd, was 5th cylinder, don't have facilities/ equipment to do a compression test at home, replaced all plugs and no difference, and battery actually shifts every time i drive it, on my list of things to do (just bought the z as a project car)

The equipment to do a compression check is cheap. You can find a tester here for $10 to $20: http://da.harborfreight.com/cpisearch/web/search.do?keyword=compression. Auto parts stores will be a little more expensive, but they should still be near that range. Of course, you would need a spark plug socket and ratchet to remove & replace the plugs, but it sounds like you already have that. As for a facility to check your compression, if you could check/replace your plugs, you obviously have the facility to do a compression check.

If your battery moves around, secure it before you drive the car again! If it shorts against the chassis, you could have some major problems including catching your wiring on fire. (Been there. Done that. Got the t-shirt.)

When you downshifted, did you over-rev the engine? What was your speed?

[slobound76]

went from 75 down to like 35, was slowing gradually, so was going about 60ish when i braked and downshifted when i got below 50, nothing out of the ordinary, revs never went above 3500 or so...will head to parts store to find a compression gauge, and yeah the battery securing is a top priority, tried a cpl quick fixes already but looks like i need a new tray i can bolt in (just purchased the car a few weeks ago, so getting to find lots of fun stuff)

[slobound76]

to clarify, the braking was fairly hard, but stayed off the gas and didn't shift til the speed was below 50, rpms stayed under 3500, so no over reving

[ezzzzzzz]

Sounds like you lost ignition. Check for the obvious such as a poor ground or loose/broken lead(s). I'll bet you find you shorted something out or disconnected. Hard braking and downshifting isn't going to blow your engine.

[slobound76]

i checked all that first thing, the distributor's spotless, all cables were clean, and replaced all plugs, which made no difference

[SteveJ]

Are you sure you have the correct firing order? Have you grounded each plug to verify it has a spark?

Also, don't hang your hat on the misfire being related to having to decellerate suddenly. You might have just picked up on something being wrong after it happened.

Have you checked your timing lately?

[slobound76]

i don't think it had to do with the decelleration, i think that something just happened to fail on the engine at the time...the ignition is good, failed spark wouldn't have oil up in and around the plug

[SteveJ]

That's why you also need to check your compression.