OK this is a 90 Canadian XJ12 but it has the ignition amp so much like an 88 XJS. Given that there's not many S3 V12 Jags, I figured that I'd post here.

Problem:

I had to take the fuel rail out to replace some hoses. When I put everything back together again I started the engine and it seemed to have a slight miss.
As it got warmer, the miss became more obvious, especially out of the exhaust which also stunk.
I checked the injectors which I cleaned through my injector cleaning machine and they were all firing but they did not sound very loud so I may have a weak signal to the injectors

I'm wondefing If I have a vacuum leak or whether I plugged in as sensor incorrectly. I had to remove the wiring around the throttle cable and put it back and the sensor on the side of the drivers bank intake manifold. That's pretty much all.

Any ideas on how to go about debugging this problem?

 

OOPS.

Loud or not so loud Injectors is not any good indication. Some of mine were like noisy tappets (almost, but not that loud), some were damn near silent.

You will have the Lucas system, as the Mare;li never appeared in the Sedans until the XJ40 and the 6ltr. That pleasure was the XJS only.

My suggestion in ALL these caes:

SLOW DOWN
Have a quiet beer.
SYSTEMATICALLY go through every item you touched, and check it twice.

I am not familiar with Sensors etc for your market, so will leave that for people that would know.

 

The 'could be' list might get a little long.

It's very easy to cause inadvertent damage while rooting around in a V12 engine bay. I'd begin by carefully examining any and all wires you touched, even accidentally. The insulation cracks off and can cause shorts. Also look at all connectors, often equally brittle.

Sometimes injector connectors don't click fully home.

Pay particular attention to the injector wiring, coil wiring, and amplifier wiring and connectors. Slide-on type connectors sometime need a gentle squeeze with pliers to ensure a tight fit after they've been removed.

Vacuum hoses are equally delicate and worth inspecting but the typical symptom of a vacuum leak is higher than normal idle speed, not roughness. An exception would be the large-ish vacuum hose at the rear of the balance tube. If not connected, or leaking, you'll get a pronounced rich mixture and rough running....and very stinky exhaust.

Cheers
DD

 

Two minds with but a single thought, Grant

Cheers
DD

 

Thanks, I was listening with a stethescope so not so loud that i would have heard them across the alley :-)

I suspect it's either a vacuum issue or maybe i connected something incorrectly. The injectors behaved fine and were all within 10ml of each other after 10 minutes of running on the injector machine after I ran them through the ultrosnic cleaner.

 

Remembered, dont rush me.

You had the Injectors out, tick.

When re-installing, the square section lower seal MUST be inserted into the manifold, THEN, the Injector, liberally sprayed with Silicon Spray, is inserted into that seal, VERY CAREFULLY.

Not doing it that way runs a high risk of that seal NOT sealing into the manifold as designed, and then you have a VAC LEAK, OOPS>

Quickie test, COLD engine, running, spray a SMALL amount of Carby Cleaner around the base of each Injector. If there is a leak, the engine will respond to that Bang Juice.

Good luck.

 

I suspect the injector seals could be the problem. I need to get a good look at the OE ones as the ones I removed looked like they were stepped and fitted in the intake manifolds. But thart could be because they were crushed.

The ones that I cross referenced them to were square o-rings. They could well be the wrong o-rings but were the only ones that I could find. Thanks.

 

Exactly.

New seals are a square-cut profile which compress into the manifold holes as the injector plates are tightened down

Cheers
DD

 

Then i probably have the right seals. That's good. I'll get some carb cleaner (or a water spray) and test.

 

So I'm still trying to figure this out when I have time. I sprayed carb clean on the injectors and do not feel a notable change in engine tone so I don't think that the injectors sealing is causing that big of a vaccum leak that it's creating such a rough running problem. I'm not trying to understand the function of the microswitches which were also moved around, they look super fragile and flimsy to the touch. I'm wondeing if they got damaged in the process.

What do these things do and could they cause the car to run rough?

 

If we're talking about the same switches.....

The microswitch on the throttle cable, just to the left of the throttle capstan, is for the transmission kickdown. It won't impact engine running.

The other, actually mounted on a bracket attached to the capstan, RH side, is an enrichment switch that closes at about 75% throttle, maybe a bit sooner. From this, the ECU will command a slightly more fueling, about 10%. You can unplug it and see of your roughness goes away. It probably won't. But takes only moments to try.

Cheers
DD

 

Yeah, plugged or unplugged, it does not make a difference

 

This board is amazing!

I'm also replacing my fuel injectors. I've got the new ones mounted on the fuel rail and ready to go. Do I spray the injectors with silicon sealant? I did not see that step mentioned.

 

Someone will correct me if I'm wrong BUT I don't think folks were talking about a SEALANT at the injectors, but a lube to allow the injectors to smoothly find there ways home when tightening down the rail as they will slide down into the rings that are pre-pressed into the holes on the manifold...?

And yes, this forum (the members) IS amazing,,, as I just learned something important here. I had NEVER don't my injectors this way and am sure I am suffering, perhaps multiple, mini leaks around injector seats. Time to dive in.

Thanks to all!!!

 

Right.

I use a product called "Sil-Glyde" for this purpose....and many other situations where an assembly lubricant is helpful

Cheers
DD