Hi Everyone!

I have a slight engine miss and I was hoping someone might point me in the right direction of how to fix it. I have an 89 XJS V-12 Collection Rouge with 67,000 miles. I have replaced the fuel pressure regulators, rebuilt the fuel rail/injectors, replaced all fuel lines under the hood, replaced the fuel pump and fuel filter, replaced the AAV, replaced all plugs and leads (gapped to .025), replaced both coils, replaced the distributor cap and rotor, replaced the fuel injector wire harness. The car sat for 14 years in storage before I bought it. I have run about 4 new tanks of gas through it since I bought it and have used BG 44k each time hoping to flush out any bad gas. Still after all that, I have a slight engine miss that comes and goes. I usually feel it when the engine is warm and I am stopped at a stop light. I noticed, the RPMs are usually around 650 when stopped at a light with my foot on the brake. I have tried adjusting the idle. I am just out of ideas at this point. I am kind of a perfectionist so, this is driving me nuts! By comparison, I have an 89 XJS convertible with 37,000 miles on it. Its engine runs as smooth as butter - no misses whatsoever. Any ideas or suggestions would be greatly appreciated!! Thanks in advance!

 

Your smooth as butter V12 is really the exception here.

Many before us have tried to get that buttery idle 24/7/365. I'm not sure that any have met with success. Back in the day, when these cars were new, road tests often mentioned an occassional tremble at idle. Jaguar itself introduced various schemes to address "idle instability" issues over the years.

You've covered all the usual bases. A subtle, intermittant fault obviously isn't easy to track down. Persistance might pat off, or might not.

Sometimes lazy oxygen sensors can cause idle quailty weirdness...although this is more commonly in the form of a very slightly oscillating idle rather than a misfire.

You might have a lean misfire condition which could be caused by a subtle degradation of ECU performance.

You might have a weak coil....most commonly this would show up after a fair bit of engine heat has built up, but that's not carved in granite.

Carefully check all relevant connections at injectors, coils, and relays. You might discover a fault. Same applies to all engine/fuel injection grounds.

Rebuilt injectors: were these flow tested so that you have a balanced set? Taken individually they might all be within spec but if you have a couple borderline injectors at one end of the spec and a couple at the other end of spec, it might be enough to cause some roughness.

Let's hear what others have to say

Cheers
DD

 

Welcome to the Forums.

When the beast is sorted, Please do an Intro in teh New Members Area.

I am with Doug on this.

Lack of use, worst thing for any car, ecpecially the V12.

One of my 4 was what you chase, and I have NO idea why it was the odd one of the fleet. The rest were a very close 2nd, and chasing that oddity simply consumed too much drinking time, so eventually, I lived with them for hundreds of thousands of miles.
No one else could pick it, but I knew there was a gremlin somewhere. I moved on.

However, trimming the ECU HOT idle fuel Potentiometer made a huge difference. I have a PDF for that if you need it.

I am assuming this is a Marelli car?? 14 Plug Leads, as apposed to the Lucas system of 13 Plug leads.

 

Have you renewed the amplifiers, the crank position sensors (one each end of the engine) and the fel pressure regulator on B bank?

 

Non, pas encore.

 

After further inspection. It appears the "miss", if you can call it that, is on the B bank. I based this off of putting my hand behind the tail pipe while running. I unplugged each injector separately on the B bank, and it only made it worse. So, the miss is very slight, I guess. I am replacing vacuum hoses now. The ones on there are 33 years old, so maybe that will make some sort of difference. Fingers and appendages crossed!

 

The size of the problem is infinitely proportionate to the trouble it causes ...

Could be something as daft as solenoid resistance, a plug defect, a piece of carbon in a critical spot - perfection is an illusion particularly in a combustion engine - have you ever actually watched a spark ? - it moves around - there may be one place on one plug where it is enough to miss on one stroke but catch on the next.

Load the parts cannon all you want the only certainty is the reduction in the weight of the wallet, if you think of 10 things the answer will be number 11 ... my money is on something in the ignition side - but then again who sorted the injectors - you ? or a pro ? - what did you use to do this are you sure of the flow rate on every single one ... can of worms ... vacuum / air leaks would tend to involve everything.

Run it until the misfire is consistent - sporadic or intermittent problems will really test your alcohol capacity.

 

Yes, sometimes the best idea is wait for (and perhaps hope for) the problem to get worse. The you have a chance of clearly identifying the cause.

Cheers
DD