heres a fun fact. its about 45 degrees F out right now and the jag will NOT add fuel to the idle mixture and its too lean to start. i got it started with some starting fluid but it only ran for about a minute before dying again.

also, on a 65-70F degree day the jag will start easily, and rev somewhat easily without bogging, but as soon as it gets up to running temps it will bog tremendously and wont idle worth a crap.

the seller assured me that he replaced the CTS, and i believe him since i know he has a fortune wrapped up in this car trying to get it to run. that doesnt mean i dont believe a CTS would be bad off the shelf.

 

The only way to check is to measure resistance. Pull it, place it in pot of water and slowly bring it to boil, while measuring resistance at the time.
There is tons of aftermarket and NOS crap that is bad right out of the box. I had to replace oil pressure sensor twice and then ended up opening new one up, to solder coil wires nicely and this finally made it work...

BTW, I was playing with my ECU's today and sadly 16CU does not work as well as original 6CU on my car.

 

thats too bad. must be why jamey sold his to me. i guess he owes you tea and crumpets.

 

Actually it worked exactly the same as the 6cu in mine. I couldn't see any difference at all. But, I figured you'd be making much more modifications for horsepower than I would, so I thought you'd appreciate the 16cu instead because it was more adaptable.
I'm okay taking it back if you aren't happy?

 

I think you should check that again. Idle vacuum should be very high on that hose, something on the order of 20-22". The MAP is the primary load sensor on this ECU, not the TPS.

Vacuum also controls the fuel pressure regulators. If you have insufficient vacuum there, you also get higher pressure on the rails and more fuel. If those are unplugged or leaking, you get overfueling at idle.

 

im just kidding bro. lol

 

If you do have a leak between the manifolds and the ECU, The leak can be divided into two sections to trace it:

Section 1: ECU to cross-over pipe at back of engine. Disconnect the flexible tube in the centre of the cross-over pipe and block it. Disconnect the vac tube from the ECU, evacuate the tube with the vac pump and see if it holds the vacuum. If not, there is better then even chance it is the flexible from the cross-over pipe, OR the flexible at the ECU end. Each of these are joined to a long steel tube that runs along the tunnel, under the heat insulation above the rear axle, and into the LHS of the trunk. The problem is that it is either impossible (or incredibley hard) to change the flexible at the engine end with the engine in the car.

Section 2: Between the manifold and the cross over pipe. There are lots of places this setup can leak. If it is not drawing a pretty decent vacuum with the engine ticking over, there is no alternative to going over each manifold and carefully blocking off/checking all the many air/vac tappings coming off each of them. For example, a stuffed vac capsule will let air into the manifolds, cracked hoses, jammed open AAV, throttle positions, etc etc.

Also do not overlook loose manifold nuts - very common.

 

the amount of vacuum at idle from the ECU is the same as the distributor advance hose. about 20 inhg or so.

 

exactly what Greg says,, is why so many poor Jags have gone to scrap/recycle yards.

JUNE 2010,, i was given a 1975 XJ12C, FOR ABSOLUTLY NOTHING,NO MONEY,000.

all because of technicians that just could not get electrical wiring,and associated parts working properly, he spent thousands of dollars, including a REBILT ENGINE.

his wife was so happy ,that i came and removed it from there property!!

and i sold it in less than a month,to a guy in MAUI HAWAI, shipping was as much as i sold it for, like i say sometimes life makes no sense!

 

Your vac at the dizzy should not exceed 11inHg. There is a vac regulator under the passenger's side intake manifold that regulates this. If you are getting 20inHg at the vac advance capsule, that means someone's been farting around in the maze of vacuum tubes underneath that manifold, and they may have screwed something up and/or left a vac leak. Buy yourself a 6' roll of vac tubing for two bucks and redo all the vacuum lines on that side of the engine. Chances are they are in rough shape and need to be replaced anyway. Mine were so soft and spongy they were sucking shut on me.

Redo all of them, even going to the dump valve, the air switching valve on the air pump, etc. My biggest problem was coming from a section less than 1" long off the thermal vac switch.

Also pull the vac lines going to the FPRs and check for any fuel - if there is fuel in the vac lines on the regulators that means the diaphragm is busted and not only is your fuel pressure messed up but you're also sucking raw fuel directly into the manifold (aka running super rich).

There are multiple sources for the correct vac diagram, you may even have one under your hood.

D

 

heres another interesting update. i pulled the ECU hose off and put some vacuum on it. about 30 inhg or so. the jag runs 100% better than it did. itll bark the tires and pick up somewhat, but not much power up top and it still backfires.
the regulators are fine. maybe i overestimated it a bit. the vacuum at the dizzy at idle is about 10-15inhg and equal in suction to the ECU hose in the trunk.

 

ive got a hose coming off the underside of the passenger manifold, inline with the throttle body but closer to the engine. about 1/4" diameter. its blocked off with a screw and doesnt go anywhere.

 

You put vacuum on the ECU or you blocked off the ecu end?

There is a small vacuum regulator under the pass. manifold. this ONLY controls vacuum to the vac advance module. So, you should NOT be seeing the same vacuum at idle at the ECU as you are seeing at the distributor. If you are there's a problem. You should be seeing ~20inHG at the ECU at idle, you should see 11, no more no less, at the vac advance module at idle.

There are about a dozen hoses under the pass. manifold, no way for us to tell which one is blocked with a screw, need to find the vac line routing diagram and start tracing out. Is the tap on the underside of the throttle body plugged? Does it have a cap on it? Wide open? If the PO removed your air pump then this could be the line going to the air switching valve, which is OK. This should trace back to the thermal vacuum valve in the back of the passenger's side coolant rail. If it's anything else, it's not OK.

D

 

the vacuum regulator your thinking of is on the firewall side of the manifold. not underneath.

and what 2 dollar roll are you talking about?!? the local autoparts store wants $40 for a 50 foot roll of vacuum hose. or $.99 per foot.

 

OK, so your vac regulator is there and hooked up then, right? That's a start. It wouldn't cause the issues you're having but at least there's not open vac lines hanging there.

$2.38 for a 6' length at my local place back around Xmas, right off the "miscellaneous emissions junk" rack. I redid all my lines and still have some left over, you definitely won't need 50' of it. If I were to pull the engine and redo everything nice and clean I probably would need that much because I'd relocate a lot of the crap elsewhere in the car (just for aesthetics).

What other stuff is there and hooked up? What about your dump valve, air switch valve (this is probably the one plugged with a screw), SAV, thermal solenoid, and the solenoid switched by the 45-sec timer?

 

Also, can you describe what you did to put vac to the ECU that caused it to run somewhat better? Did you plug the crossover port and connect that line somewhere? Just stick a Mityvac in the trunk?

 

i unplugged the ECU vac line at the spot where it comes out of the trunk and then sucked the air out of it until i had put about 30-40 inhg of vacuum in the line, and capped it off. this put negative pressure on the ECU baffles. then it idled fine and had a decent little bit of power.

the line coming from the crossover to the ECU is fine. theres no leaks at all in this line.

ive found no other vacuum leaks. the issue is strictly fuel at this point. when i charge the ECU hose to about 30inhg, the ECU is leaning out the mixture. so something else is making the ECU add too much fuel.

im starting on the injector harness.

 

Stupid question but you did cap the line coming in to the trunk while you ran it? Any chance it was sucking air into the crossover, and that was what enabled you to run?

In the end I think you're right to start in on the injector harness, good luck.

 

yes. i capped it as well, but it didnt make a tremendous difference in the way the car ran or idled when i left it uncapped.

 

heres another fun fact. after letting the jag run for half an hour or so, playing with vacuum hoses etc. and making baby step progress, i finally got the car to a point where i could plug the ECU vacuum hose back into the crossover and it would idle! it even ran and everything. at this point the engine was pretty warm.

drove the car up and down the driveway and it drove ok. parked the car, shut it off, fired it back up and it still ran decent. shut it off, went to start it back up and realized the positive battery cable had come undone. reconnected the cable, fired it back up, and it wouldn't run with the ECU vacuum hose plugged in. i disconnected it and manually put a vacuum on the ECU an it ran fine after that. couldnt get the ECU to run off the idle vacuum anymore.

it seemed like the ECU had finally adapted to run at idle vacuum, and when the battery came undone it went back to square one.