83 XJS V12 that I just put an 85 motor in. Motors are virtually identical and I switched the intakes and everything off the original motor.
Now here's the problem: The engine runs rough, smokes terrible, very rich. I suspected a vacuum leak and found that if I attach an electric vacuum motor to the vac-line on the computer in the trunk, it idles beautifully. (I'm assuming that vac line is how the computer adjusts lean/rich mix.) Once the motor is running good, I have good vacuum, but if I switch from the electric vac back to the engine vac, it runs rough again.

I can't find or hear any leaks and I'm guessing it must be fairly good sized to create this scenario. Where are the most likely places for leaks? I've installed all new intake manifold gaskets and am reasonably sure they aren't leaking. I've plugged off just about every line I can find except for the brake booster and the reserve tank. Any ideas?

 

I assume you've checked the the vacuum line running under the body of the car isn't broken and is attached to the balance tube, yes?

Sorry, but I had to ask.

Cheers
DD

 

Yes, I checked that and it's good. Oh, and I've also plugged off the AAV...

 

Smoke can be a number of things.

The "new" engine is a smoker, or has sat a LONG time, and needs running to clear it up, usually a "blue" tinge to the smoke.

The vac modulator in the trans side is fractured and it is sucking ATF up, and smoking, usually "white" smoke.

The master cylinder is leaking brake fluid into the vac chamber and the engine is sucking it up, white smoke again.

With the AAV plugged the idle should be at about 400rpm, so any more with that plugged is a vac leak.

Put a vac gauge on the engine, any convenient vac spigot, and make sure it has suitable vac supply????. The fact you can sort the running by simulating vac at the ECU has me thinking the engine aint so bright.

I did this way back using a "T" in the boot vac line to the ECU, and found near zero, and a plugged vac pipe under the car up in the trans tunnel, bugga of a job to find and fix. That is why I suggest using the ECU line, coz you just had the trans out, and maybe nipped this pipe on refit????.

 

The engine only smokes when there is no vac applied to the computer. Once vac is applied it runs smooth and clean. On it's own, vac is running at 17 and from what I understand it should be 22-ish.
I figured my next move (other than check timing again) is to plug off the brake booster and the vac tank. Then the only other thing receiving vac is the PCV valve (which I checked and is good).

 

Interesting problem. IMHO, 17" vacuum at idle isn't excessively low

I'm still on my first cuppa coffee but I think we have a chicken or the egg situation. Is low vacuum causing rich running or is rich running causing low vacuum?

In addition to the checks and experiments already mentioned make sure the fuel pressure regulators are seeing that 17" vacuum. Or perhaps isolate the fuel pressure regulators and apply vacuum directly to them....and see what happens. Would only take a couple of minutes and can't hurt, eh?

Cheers
DD

 

I attached the electric vac directly to the inlet side regulator, but there was no change.
I do have a thought though...there are 2 microswitches on the throttle linkage. One changes idle mixture at start up, the other is for full throttle. What symptoms would there be if I had their connections mixed up?

 

Check your charcoal canister area, under driver's side headlight - fender.
I would plug off all 4 vacuum lines that go in there, it does not do anything anyways. Mine was leaking vacuum. Also, I had some leaky lines under passenger side air cleaner box, I plugged them off as well and my idle went down to normal level.
Normally, when there is vacuum leak, motor idles at 900-1100 rpm and AAV does not regulate it, however what you are describing, might also be faulty vacuum sensor in the ECU, that requires a lot more vacuum to actuate?

 

Try the outlet regulator, just for the heck of it. That's the one that controls fuel pressure, really.





Maybe I'll learn something new here but I think what you should have is one switch on the throttle capstain (enrichment) and one on the throttle cable (transmission kickdown).

The enrichment switch, which by the way is wired in series with a vacuum/electric enrich switch, will richen the mixture about 10%. Most engines actually like this bit of extra fuel at idle but if you're already rich for another reason the extra 10% might put it over the edge.

Anyway, disconnect both and see if anything changes.

The enrichment switch should have black and white/slate wires.

Cheers
DD

 

Hold the phone...
The enrichment switch is wired in series with a vac/electric switch...?
This engine was partially disassembled when I got it. What/where is this vac/electric switch and what does it look like?

 

In parallel, not series, sorry

Cheers

 

so I'll have to find the vac/electric in the pile of parts and figure out where it goes...

 

Eventually, yes, because you really should have it. It mounts at/near the rear of the RH inlet manifold. It's blue/white plastic.

Not having it it will not be the cause of your current problem, AFAIK

Cheers
DD

 

Okay, found it, and had it already inline, I just forgot about it. Now, the microswitch on the throttle capstain is the throttle enrichment? It doesn't "click" until almost full throttle, does that sound right? The switch on the cable clicks as soon as the accelerator pedal is moved.

Either way, I disconnected them individually, then both with no change. I also tried it with the switches triggered...nada.

Now, here's a question that should boggle the brain. Is it possible that the injector timing can be off when the ignition timing is on? The reason I ask is because I was afraid I was flooding it, so I disconnected the injection harness and turned the key and it purred like a kitten until the excess fuel was gone and stalled. Since I'd just rebuilt my harness I wondered if I messed up the wires, so I reinstalled the old factory harness and it acted exactly the same.
Now, considering I can produce the same "good running" effects by applying vacuum to the computer, I'm left feeling confused and disoriented!

 

Yes, on the capstain, right-of-center....enrichmentment switch




Yes







OK, this is this switch mounted ON the throttle cable where the little actuator arm fall ito a hole on the cable casing, yes? That's the trans kickdown switch. It shoud close at wide open throttle.

But that's a different issue.....








OK, good






Ignition timing and fuel injector pulse are independant of each other. The only ignition function the fuel injection knows about is rpm....from a trigger wire coming off the ignition amp.

The injectors are batch fired in four groups of three. The amount of fuel injected is dictated by pulse width (how long the injectors stay open) and fuel pressure.






Well, thinking out loud, let's look at how an over rich mixture can occur.


- Fuel pressure too high
(faulty outlet regulator, no vacuum to regulators)


- Leaky injectors
(drip drip drip...maybe a seal problem or clogged pintles being held open)



- Injectors staying open too long, either by an external fault or by ECU control
(external fault might be a wiring problem but you seemed to have covered that. I wonder if a fault in the injector ballast could cause injectors to stay "on"? Hmmm. Have to think about that. As for ECU commanding the injectors to stay open too long, well, there are multiple possibilities for that: coolant temp sensor , TPS, etc.)



Me too :-)

Is lack of vacuum a problem or are you masking a different problem by artificially increasing vacuum? Perhaps the ECU interprets high vacuum at closed throttle as "over-run" and cuts of the fuel entirely? I dunno. Just pondering.

I'm not really sure yet (or perhaps ever) what the root cause is but, personally, I would assume rich mixture is the main problem and low vacuum is a secondary result...not the other way 'round.

Others will chime in. One way or the other it'll get fixed :-)

Cheers
DD

 

Okay, here's an update:
My friend and I figured we'd at least get the timing done while we could make it run smoothly, and I found it was running at about 20 degrees off with the distributor turned all the way. I figured the plug wires were 1 off on the dist-cap, so I moved them over one spot. Now with the distributor turned all the way the other way, it was completely off the scale. Hmmmmm....so I checked the centrifugal weights and found them jammed. (Piece of dirt or rust?) I cleaned, reassembled and hopefully that will fix it. I won't know until I get back Tuesday afternoon.

 

So I've fixed the distributor, checked and double checked the timing and I know I have it about where it needs to be. I cannot find a vac leak anywhere (with one possible exception) and the engine has excellent compression. The best vac I can get is 19 and it won't run smoothly with vac less than 21-22.
The crossover tube has metal tube inserts that the hoses clamp to and they're loose. I don't think they're loose enough to cause the symptoms I'm seeing, but I've sealed them into place and will try again in the morning.
If anyone has any other suggestions I'm willing to listen!

 

I agree on those tubes, they are not causing your issues in my opinion.

Have you tried trimming the fuel pot of the ECU. It may be at the full rich end of its 15 click scale. Mine will produce black smoke if I turn it all the way to the rich end, and I dont remember which way is what, it has been many, many years since I even touched it.

The only other item I can think of is the vac fuel sensor "inside" the ECU is having issues, which is not common, but at the same time not unheard of. If another ECU is available try it, just for the diagnosis elimination.

An '83 car will possibly have what is known as a 6CU ECU, and they did have fueling issues, my '85 did way back, and I aquired a 16CU, from about late '87 onwards car, plugged it in, set the fuel trim, never been back there in 10 years.

 

Trimming the fuel pot? Hang on...what/how/where is that?

 

that gave me a clue and I did a little research...found it and have a few things to check including a harness ground that can cause it to run rich!