teke184

continuing problem.

car runs fine and gets good gas milage. you can drive it for weeks as long as you don't runn past 4000rpms, otherwise it starts misfiring on cyl 5 and 7. i'll explain more later.....

i've tried plugs, wires, coils, injectors, swapped valvesprings.

i did the valvespring swap thinking it was a weak spring and i was getting valve float.

took it out for a test drive after the spring swap. slowly increased the rpms (2nd gear) untill it hit 4000 and shifted and came back to normal cruising speed. no problems, pulled hard and sounded good.

took it out for drive #2, and did the same thing, but this time went to 4500rpms or so, then returned to nomral speed/revs. 30-45 seconds down the road, it started misfiring!!!.

and when it starts this, you can be sitting in the driveway idling...while it misfires like this:
miss for 10 sec....smooth out for 5...miss for 15sec...smooth out for 10...miss for 5sec...smooth out for 5...miss for 15...smooth out for 5....

it starts and stops missing like this for quite a while....sometimes for 20 mins then stops totally...sometimes for an hour...then stops!!

what could give this kind of delayed reaction in the miss?? i can scan it and it has always been isolated to cyliinders 5 and 7....7 being the highest misfire counts.

angel71rs

I've seen a similar problem on a sbc. Ran fine riding around normally, but getting on it resulted in misfire + wisps of white smoke. It was a head gasket. Sealed ok just normal operation, but high cylinder pressures caused it to give. Then it would settle back down to normal after a while. But it was an easy diagnose. The 69 Camaro it was in had dual exaust. Smoke came out only one side.

On high miles sbcs (can you tell most of my experience is old school?), hydraulic lifters could cause something similar. They would stick in the bores or internal valving mechanisms wouldn't work properly due to gunk. Ticking was sometimes the tipoff, but it was light ticking. Common fix was to run a quart of Rislone with an oil change to untsick the lifters.

5 & 7 are next to each other, so an intake gasket leaking could do it too. Try retorquing. Where is the vacuum hookup for power brakes on these engines? If it is on intake near 5 & 7, a leaking booster might lean out those cylinders. If it does hook up in that area, see if you can pinch off the line to the booster with needle nose vice grips while misfire is evident to see if goes away. Pinching off the line to the booster is my favorite method of diagnosing a leaky booster.

Look at wiring too. I've seen engines give problems after being taken to WOT where the engine rocking over strained wires. But they didn't go back to good on their own like yours does.

teke184

finally some possibilites....thanks...

i'll see what i can do with those suggestions.

as for the head gasket, there is no white smoke, just the smell of unburnt fuel...although with no cats i smell that alot

one time i tried some oil flush, and with in 30 sec it was running fine again...but i changed the oil, took it for a test drive and it immediately did it again

AL SS590 M6

Vacuum line for brake booster is in the middle of the back. It would lean out 7&8.
Coils are on tappet covers so there is zero strain on the wires at any time.

AL SS590 M6

Teke,
Check your fuel system. Make sure that you have the right fuel pressure at idle and at WOT under load. Then check for injector bleed down by shutting it off and seeing if the pressure falls.

teke184

thanks al

but would a problem with the fuel system show up AFTER a wot run? and persist at idle for an hour?

AL SS590 M6

Don't know, just another thing to check and rule out.