Engine Misfire and Power loss 1987 VDP 4.2 US

Hello All,

Recently my car developed a “missfire” (almost overnight). Car was running well. I have power loss and shuddering in the engine that is consistent. It shudders at idle and it sputters to get going predictably. It doesn’t stall. Feels like its running on 5 cylinders.

Number 5 Cylinder seams to be the culprit. When I pulled the injector plug out it was the only cylinder that there was no change in engine performance. The car declined in performance after pulling any of the other injector plugs. All ignition lines were firmly in place.

Next I tested spark with a test light in all lines and have positive result there- all cylinders flashed the test light.

Next I put a noid light on all the injectors- noid flashes on all cylinders.

Next I pulled the plugs and number 5 is comparatively darker than the others. Running the car showed a clear spark. When connected but pulled out and sitting on the top of the engine.

Next I used a stethoscope on the injectors and can hear a clear and consistent “ticking” in all injectors.

Any ideas as to what might be going on here or what I can try next? I guess spark plug gap but that doesnt really explain why this would happen over the last 400 miles of driving.

Ignition system (wires, cap, rotor, plugs amplifer are all new) along with all new injectors, fuel pump, changeover, filter and gas tanks. All under 1000 miles ago. Car has 53K miles on it.

Thank you in advance!
Jeff

Jeff,
I recommend that you do dry and wet compression checks of all the cylinders. Then remove the camshaft covers and check the valve clearances for all of the inlet and exhaust valves. The compression check would confirm if the source of the problem was due to valve or piston issues. Checking the valve clearances would confirm whether of not the clearances are within the 0.012-0.014 inch spec for your engine.

Paul

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Jeff,
I had a similar problem about 20 years ago in my 1984 XJ6 Vanden Plas. When I did the compression check, #1 cylinder was only about 20 PSI while the others were all around 110-120 PSI or so. It turned out that I had a burnt #1 exhaust valve and the clearances on other valves had all also closed up a bit. Seems to me I had about 90K miles on the car at the time. I removed the cylinder head, had a machine shop rebuild the head, and then I reinstalled it. I drove the car for another 15 years and maybe 100K miles or so after that. This was a valuable and expensive lesson for me to learn about the importance of regular valve clearance checks and the necessity of adjusting the valves if they are out of spec. Otherwise valves can burn and expensive repairs will follow.

Paul

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Fairly complete diagnostics.

But, for one!!! Wet and dry compression tests. I have a feeling, rather strong. # 5 will b a lot lowermthaan the others.

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Like other have said, Jeffrey; compression test!

It will quickly reveal whether the problem is within the engine itself - or something external…

Frank
xj6 85 Sov Europe (UK/NZ)

Hello Gents!

Compression test seams to be the unanimous. Since I know i had consistent strong readings prior to this, it should be instant! I am moving houses and my garage is a nighmare- the hardest part will be finding my compression test kit lol.
Will report back! Thank you!

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Circa 1955, I drovew from NY city to El Paso, Texas. Just released from army service. drove my Belgian assem,bled 50 Chevrolet. Somewhere along the way it developed a low RPM miss. But, was OK at highway speed. I knew that a sustained 60 was the best this engine could handle.

Once home, other issues were of more import. Like landiong a job. After i checked for spark and valve adjustment, the answer was easy, I did nt have a meter to check compression/ In my stash at my parents home, I had a spare head!!! I had it serviced. Got a gasket kit., And in the driveof our new cracker box house began the swap. Done one Saturday., Fired it up. Sweet. All six at work, Nice smooth idle. A few weeks later, i got past my probation at my new job. Was issued a company car. bye bye 50 Stove bolt. . t

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I think it is pointless to do any further work until compression is verified, Jeffrey - good luck with the moving, which should have some priority…:slight_smile:

Frank
xj 85 Sov Europe (UK/NZ)

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I except and agree with the compression test tappet clearance comments , however for the simple job at little cost , put a brand new spark plug in #5 . Your tests have not eliminated that in my opinion . Normally burnt valves , ect , tend to loose power more gradually , not over night. Regards Michael

Hello All,
So compression checks out in all 6 Cylinders. New spark plug had no effect. I am not surprised at that but worth the try. All the other plugs looked clean except for cylinder 5. So I’m a bit stumped. I guess i could still pull the valve covers off and check clearance. Maybe a leak-down test is in order. Any thoughts?

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Not likely to provide any useful information. if the compression in 5 is similar to the other cylinders, that is not the issue.

Check for spark at number 5.

  1. Attach an inductance driven strove timing light to that lead.
    crank the engine. No light, no juice!!

  2. Attach that lead that goes to number 5 to a spare knownm good spark plug. Gap it wide. Make sure the base is well grounded. Crank the engine,. Fat crackling blue spark across the elctrodes? Wimpy yellow not good enough NO.

proable bad lead, or dirty terminal or not well seatedin the post on the distributor cap.

If all OK, it is an injector issue…

Carl.

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If it’s not a weak spark: I’d swap the the injector with another cylinder. I was suspecting a leaking injector. Being new doesn’t mean they can’t have issues. Good troubleshooting, thank you.
David

Swapping the injector was my next test! If it’s dumping we’ll know instantly- just a pain pulling those things out.

You could also pull the injector

Locate the bullet connector running alongside the injection wiring loom, from the coil to the multi plugs

Turn on the ignition, make and break that bullet connector, this will fire the injectors every 3rd time (very sensitive, don’t count) because it fools the ecu into thinking the engine is running, see how it sprays. May be a good idea to disconnect the other injectors so you don’t drown the engine.

To keep the pump running push open the AFM flap a little, hardwire the pump or jump the relay. Or “crank” in D or R if you have a helper, that runs the pump but not the starter. Otherwise you lose pressure after a few taps.

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Or dumping no fuel. Stuck shut.

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Hello, you did check the injectors so that would not be the cause. I would have guessed that this would have been the problem. I had a very rough running engine and thought I heard all injectors clicking. After I changed ignitioon wires and distributor and rotor, spark plugs also. I checked the injectors again. Although they seemed to all make a clicking noise some sounded harder than others. So I removed 3 of them that were suspicious. I checked them and I saw that they were kind of stuck. I used WD 40 and later an injector cleaner to get them working again. My car was sitting for a year.
I could not think of any other problem.