88 XJ-S V12 HE running rough

The car is running rough. More so at idle but rough at all rpms. Idle is steady at 750 in P and 600 in D. It accelerates fine with no hesitation. Fuel pressure is 36 and all injectors are clicking.
The right side exhaust has a strong pulsing. The left side is more continuous which leads me to believe that the right side has a miss.
Can I check vacuum at the rear of the right side intake manifold?

Hi Marvin, This is not an answer to your question…. Have you adjusted the throttle plates, using a feeler gauge, As described in Kirby Palm’s book? Looking for a rough idle solution before following this procedure is futile so I just wanna make sure you already did it. -John

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I cleaned and checked the butterfly valves and they are at .002. I’m thinking vacuum leak as it’s only reading 11in at the rear of the right intake but I thought a vacuum leak would cause a high idle.

BTW I’m at 4,800 feet altitude so I would expect the vacuum to be 4-5in lower than the manual.

Suction is strong on the left air cleaner intake. Putting my hand over it causes the idle to drop. Not much suction on the right air cleaner intake.


Oh dear. Done a compression test on all cylinders? Not trying to scare you, but trying to rule out some of the classic faults.

Vacuum leak should be fairly audible. You can also get one of those harbor freight smoke testers, then the leak would be visible too. But… a leak massive enough to have uneven suction between banks? Hrmmm

The right side, being the USA passenger side, does not have the AAV. You’d think if the problem was a stuck open AAV you’d have less suction on the left (despite the interconnection of the banks with the rear crossover pipe)

Might be as simple as an ordinary secondary ignition misfire: faulty plug wire, fouled plug, that type of thing

Or a clogged injector or two.



Misfire or clogged injector. It would explain the low vacuum as well.
Try to identify which cylinder by unplugging spark plugs and injectors.
A vacuum leak would cause a high idle.

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As the previous two posts, perform a cylinder balance test to determine the low performing cylinder giving the phut exhaust sound.
Disable the fuel injection system and crank the engine to listen for uneven cranking (giddy up) due to low compression when cranking.
If you have an oscilloscope, perform a cranking relative compression test by monitoring battery voltage dips while cranking.
Use WOT for cranking tests.

I suspect either a faulty HT system, or stuck injector.
The injectors fire in four sets of three, so it’s unlikely to be a significant electrical issue, as that would disable three injectors.

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Check vacuum lines on fuel pressure regulator to see if it’s sucking gasoline…it shouldn’t

Wonder what the ECU is doing to A/F mixture at this elevation ? O2 at sea level is 20.9% while at 5000 feet…it’s 17.3% Has your vehicle operation always been at 4800 feet ? O2 at 5000 feet is 17% less than at sea level. SD Faircloth

FP vacuum lines are dry.

I ruling out a vacuum leak because:

  1. Idle is steady at 750
  2. If normal vacuum at sea level is 14-18 then at my altitude of 4800 11in is not out of the ballpark since vacuum drops about 1in per 1000 feet.
    Vacuum lines to FP regulators are dry and pressure is 36.
    I don’t think it’s an ignition problem since it’s only been 7,000 miles since new
    plugs/wires, distributor rebuild, and injectors redone. Also, all injectors are clicking.
    I’m leaning towards a clogged injector since the roughness began when I started it after winter hibernation. I going to put in some fresh gas, ethanol treatment, and a fuel system cleaner and run it a while to see there’s improvement.
    I assume that if I pull a wire off an injector and there is no change then that injector is suspect?

I have never operated a vehicle at 5000 feet, therefor have never had to consider A/F ratios at an altitude greater than say 100 feet. If the air has 17% or so less O2, then I would suspect the engine is running rich…unless the O2 sensor feedback can accommodate that much correction. We don’t know anything about your vehicle. Has it always lived at 4800 feet or is this recent ? How long have you owned the vehicle ? How long has it run rough ? Just started, or months ? Do/did your friends or neighbors have engine issues if just going to 5000 ft elevation ? Have you tried adjusting AF ratios with the screw on the ECU ? How old are the O2 sensors ? SD Faircloth

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Unless that cylinder isn’t contributing for another reason, such as no spark, or compression.

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Where you at? Denver?

Salt Lake City

I’ve had the car for 14 years and it’s always been at 4800 feet. It started running rough when I woke it up from winter hibernation which is why I’m thinking bad gas or clogged injector.
As for A/F ratio, with carbureted cars you usually have to change the jets but with fuel injection the ECU should automatically compensate. I’ve never heard of a fuel injected car needing adjustment for altitude.

OK. Then as others have indicated, check each injector and plug on the right side of the engine, see if they are operating. That’s the easiest place to start, albeit it may not give a definitive answer. If you don’t have a mechanic stethoscope, a long thin metal rod or wooden dowel will do fine. SD

I checked each injector with a stethoscope and they are all clicking. Started removing the injector wires and pulling the 2nd one back from the front made no change. Before I started tearing things apart I decided to drive it some more with new gas and plenty of injector cleaner. After about 25 miles it the roughness went away so it must have been a clogged injector. There is still a slight intermittent miss but nothing like the strong steady putt-putt I was getting.

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Dieselman, I believe is on the right track. A $200 (or less) oscilloscope, such as the Uscope or the least money Pico scope maybe the most efficient and quickest way to get to the issue. I’m surprised more of us don’t mention that these issues we are discussing are best solved with a scope.
This Utuber, Drivelinemaster, has a few interesting short Uscope video’s.
uScope Relative Compression Testing Using Voltage - YouTube