Any ideas?

[richyg]

i did a compression test on the cylinder that isnt firing on my 5l and it was 120lb bang on (should it be between 150-250lb? is this serious?) The plug smelt of fuel but wasnt soaking. the engine has had new plugs, leads and coil and also dizzy cap and rotor arm. I havent actually checked for a spark, could have done with another pair of eyes, the only way i know how to test spark is to remove plug, fit back into lead and fire engine for a sec while somebody watches. (is there another way?) is it safe to leave engine running with plug out so i can run round and look?

if it turns out there is a spark, is there anything simple that could cause the cylinder not to fire before i go down the camshaft, piston rings, full rebuild road. Maybe a fuel injector problem or something? Now that i have topped up the oil and replaced the exhaust manifold gaskets, the engine is sounding pretty sweet with no nasty noises or tapping.
However, if i manually open the throttle very quickly the engine stalls before increasing in revs.

Any light shined on the matter would be very much appreciated.

[chassis 33]

You can tell if you have a dickie injector by disconnecting the lead to it, if the engine develops a further missfire/get worse then the injector is behaving. Don't do this to all EFi engines though eg later GEMS/Bosch Landies/Rangies, you can knacker the ECU.

Regards
Iain

[poorpeet]

It may be tracking although I doubt it as you have changed most things. Have you changed the metal plug extenders? If not take it & the plug off clean & dry the plug & put if back together minus the extender that should rule it in or out as a cause.

[richyg]

hello chaps

Cheers for the suggestions. Have'nt had time to get under the hood so far but hopefully will on sat! Btw,forgot to mention that i have fitted new extenders also. Will try dis-connecting the missing cylinders injector and also will double check for a spark.

Before i bought her, she was sat for a year doing nout, this may have something to do with it

[tyre_tread]

the value of a compression test is in the compariosn between cylinders and althouugh your expected figures may be 150-250, if they are all 120 it could be just poor sealing valves.

Do a compression test on all eight cylinders. They should be within 10% of each other.

If one cylinder (or more) is significantly lower than the rest then do a "wet test" - this involves putting a cap full of engine oil down the cylinder bore and doing the compression test again. If the compression improves on a wet test teh the problem lies in the poor sealing of the piston rings. If it doesn't improve then probably a valve problem.

CAUTION - forcing unburnt fuel and oil down the bores can ruin the catalytic converter.

[shpub]

Fuel starvation? Blocked fuel filter? Pump on its way out? Easy to check with a fuel pressure guage.
Does the ECU show any fault codes?
Duff throttle pot?
Duff temp sensor?
BAD AFM signal?
the list is fairly endless...

[TVRmuppet]

Swap the spark plug to another cylinder and see if the problem moves. The plug may well spark under no compression but not under load. This happened on my Lotus even though the plugs were new. Its a simple way of eliminating the plugs.

[spend]

OT. A spare king lead is a very useful spare to carry IMHO., since you've replaced your leads - stick the biggest one in the boot so its handy.

Dave

[poorpeet]

Just had a thought. Are you sure it is only missing on one pot? If it were missing on a couple it could just be that two ht leads have been put on in the wrong firing order.