I suddenly have a constant misfire on A bank, both in P or in D, using paper on exhaust pipe test. Harsh constant popping. I pulled plug wires one at a time and the idle got worse on all except 4A. So I swapped wires with 4A, no change. I pulled spark plug on 4A, looked fine. I swapped spark plugs between 4A and 1A, still points to issue at 4A. I listened to injector on 4A, constantly ticking. I get a good consistent spark to 4A.
I had injectors professionally cleaned last year, and i did a compression check last year, all good.
What do i look at next?!
All i can think of, is something is suddenly up with injector 4A. Without a scope, all i can think of is to swap injectors with another cylinder and see if the misfire moves.
Coincidence or not? 4A injector is the only one that has lost its protective boot. The wires look ok, although I āmayā be able to see wee crack? If i was getting a short, wouldnāt the injector just not work?
Good idea (actually swapped injector connectors between 4A and 5A, same thing). 4A still no difference in idle when pulling spark plug wire.
So injector wires ok? This means either bad injector on 4A, or cylinder/exhaust valve issue on 4A? Im hoping its just injector?!?
Should I test higher rpm to see if misfire goes away? Perhaps some injector cleaner? (Ironically, i just ran a bottle of Techron previous tank)
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
4
There is some possibility that junk got in there and plugged up the screen in the injector inlet. But I agree with your conclusions, either a bad injector or valve/cylinder issue.
The injector is clicking open/closed which is a fairly good sign, but obviously not always definitive. I would suggest a compression test before pulling the fuel rail to test/swap an injector. SD Faircloth
Iāve done compression tests on this car before, but did it on all 12 cylinders at once with all plugs out.
Can I do just 4A alone, so I donāt have to pull all the other plugs? (I of course disconnect coil wire, and pull fuel pump relay)
Read in the archives, I should also check the intake manifold around 4A, in case air is leaking in?
I read in the archives that a āpoppingā from exhaust is ignition, and mine is definitely popping. But I āthinkā iāve proved itās not ignition?
More info from memory, Iāve always had an intermittent miss on A bank, but now itās constant. Iāve recently replaced downpipes with more exhaust breathing. Any relation?
Do a leakdown test on 4A. This will be the only plug you will have to remove. Compression test with 11 plugs still in could be misleading - the starter will not be able to spin the engine as fast and you could get a number that doesnāt match what you saw before.
Thanks, but not only have I never done a leakdown test, I donāt have the compressor/tools to do one. Would be easier for me to remove all 12 plugs
Just curious, as Iāve done two compression tests on this car last year, and all cylinders passed with flying colors (all very high from 210-225) how likely could an exhaust valve suddenly go bad? Car has 52,000 miles.
Iām trying to go through all other possibilities before assuming injector or compression is at fault. Car otherwise has tons of power, and runs well, no smoke from exhaust.
A slight misfire is not going to be apparent on a V12. Only someone willing to do the $100 bill test described in āThe Bookā is likely to find it out :-)))
I would not pay too close attention to the mileage. The car is 30+ years old and under your ownership for about 2 years, with much of the driving happening in the last 2-3 months. I wonāt be surrpised if you pul the plug and discover that the ceramic has cracked (there was a thread of yours not too long ago covering something similar)
Did some more testing this morning. I swapped the spark plug again with another cylinder. (Yesterday I swapped with 1A, today I swapped with 1B). Still misfire at 4A. I even cleaned out the threads and mating surface at 4A, in case the spark plug was not making a good ground or not seated well. So I have to assume itās not plug/wire related. When I pull the injector plug from 4A, at idle I can tell no difference, just like when I pull spark plug wire. (But remember, I can hear 4A injector clicking away)
I also tried advancing/retarding the timing for the heck of it, no difference.
I tried a higher RPM (about 1400RPM), and the miss is still there constant at every stroke.
Hereās a possible clue, I triggered the rich mixture switch (when you are flooring it) and at idle, the miss was still there, but wasnāt constant, it was like 80% misfire. (didnāt get a pop every single stroke)
So iām leaning (no pun intended) towards the injector possibly clogged or not spraying well? Like I said, I had them cleaned last year, but who knows what could have happened in a year.
Unless anybody has some other ideas, iām going to run some heavy duty cleaner (seafoam?) through the tank and see what happens. After that, Iām pulling the fuel rail (I was going to this summer anyways to paint it), and send off 4A injector to Faircloth (he did my injectors last year) and see what he finds.
And is it a coincidence that I just ran a bottle of Chevron Techron a week ago? Could it have ācleanedā something in the fuel line and clogged an injector? I also just replaced the fuel rail hose (between FPR and fuel rail) because of that rubbing gash on the other one. I swapped in my āusedā one that still looked good, but has been sitting around for 1/2 year. I was very careful to keep it clean around there.
yeah, it could beā¦but my compression was awesome last August when I last tested it. I guess something could have happened since then. I guess itās worth a test.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
13
There is one other possibility and that is thereās something amiss with the distributor cap, sending the 4A spark to ground for some reason.
Youāre right, worth checking as itās easy enoughā¦am I looking for cracks or carbon build up on that post underneath? Will do that tonight and report back. Iāve also added some B12Chemtool injector cleaner to a full tank, will see what happens over the next few weeks.
Side note, my car has NEVER had a smooth idle since I bought it almost 2 years ago. Have had random misfire from both A and B bank over the last year. Using $1 bill test, now B bank is perfectly smooth, and A bank has constant misfire, which seems to be 4A. Something is amiss.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
15
Donāt forget to check the HT lead to the 4A plug, too.
Yeah, i was thinking about vacuum leakā¦i tightened bolts around 4A, and sprayed some soapy water, but cannot be 100% sure itās perfectly sealed. Iām reluctant to spray carb cleaner near there as I did that last year and got a large BANG which scared me, strangely put a dent in the can, is there a safer way without starting a fire?
is it easy enough to pull intake manifold and replace gaskets on that side?
I want to add, over the last week or two with this misfire, Iāve noticed the ECU is suddenly running very rich, and Iāve had to dial it back leaner to get numbers near to 1.9V. Iāve never had to run the ECU this lean to get the right numbers. ???
Could my new exhaust, with a better breathing engine, have made a bad intake manifold gasket get worse because itās sucking air faster than it could before?
Greg, Send me an off list e-mail. sdfwinddancer@aol(dot) com Used (dot) so a link doesnāt appear. I am still ignorant on replying to e-mails sent thru JL to me. Donāt know how to answer them direct to the patron, and not going thru JL. Cheers. SD Faircloth
One last thing popped into my head - over a year ago, I removed my smog air pump and plugged up the holes that go through the intake manifold to exhaust.
Now with my exhaust flowing better with new downpipes, could that have forced some exhaust air up one of my plugs that may not be perfect, and perhaps exhaust is leaking past intake manifold into intake?
