Initially had the DAC3586 (84253A) 6CU installed in my 1986 XJS V12. It would not run right with the vacuum line attached or unattached. I purchased a DAC6337 (84787) 16CU as an upgrade but I'm not sure if the compatibility is right.

Was trying to sort out some stumbling idle and stumbling acceleration issues with the newer ECU installed. On a whim, I installed the old ecu back in place to see if it made a difference. The car ran great... for about 10 mins until i pulled in to the parking lot to get my hair cut. After that, it started exhibiting the symptoms that made me replace it in the first place (no starting, no acceleration, strange things with vacuum attached/unattached). Regardless, I installed the DAC6337 back into the car to make it run "reliably" bad. I know where I can get another 1986 ECU for probably a good price. My question is: Is it a compatibility issue, a bad "new" ecu, or something else? any input would be helpful.

Thanks,
Jason

 

gonna try some other random things... anyone tell me where the coolant sense line is on the A bank that you can disconnect to defeat the 15 min warm up timer? I looked all over and i only found a little module that looks like it has a wire and a vacuum line attached to it...

This is getting pretty frustrating... but when it runs right, it makes me want to get it fixed that much more. I KNOW it can run right, i just cant make it happen.

 

My gut feeling...and that's all it is.....is that the ECU swapping is masking a different problem.

Could you be a bit more specific in describing the symptoms you're chasing?

Meanwhile, go to the 'How To' thread at the top of this section. Select Downloads, then Tech Guide, then S58 Engine Performance. Lot's of good info.

I'll check back tonight

Cheers
DD

 

Thanks Doug. I'll get the Tech Guide.
Symptoms are different depending on which ECU i have installed.

Full disclosure:
Car sat for at least 2 years i believe
replaced: Fuel pump
Fuel filter (several times)
Fuel pump relay
ECU (back and forth as described)
Fuel pump pickup filter
Distributor cap
spark plugs
spark plug wires
pickup coil
Ignition coil(went to single coil instead of dual)
Inner portion of ICM (the HEI Module with the gm one)
TPS with Mustang one
full throttle micro switch and trans kickdown micro switch
coolant temp sender
aux electric fan
rear brake rotors/calipers
front left wheel bearing


Things i've done:
timed at 3000rpm 18deg
coated fuel tank
bypassed RH FPR
checked vacuum routing
verified vacuum at ECU
AAV doesnt seem to be leaking
Cleaned out the radiator at a radiator shop (recommend to all)
Ran quite a bit of injector cleaner through
checked TPS voltage
checked fuel pressure at rail
cleaned every connection and ground i could find (doubt seriously all of them)


There may be more, but that's what i can think of. Other things i've noticed that may/may not have a bearing on what i'm experiencing:
trip computer is spotty at best (all seems to work except the fuel consumtion, seems high and the instantaneous bounces from 0 to 99, randomly resets the clock)
with new ECU installed and stumble idling, i can hear the injectors stop clicking as it falls down in rpm (like it's hit overrun or something)
both ECUs run very rich with vacuum line detached, as it should
varying vacuum on new ECU at idle seems to have no effect on stumbling

Installing the new ECU is the only way to get it to drive. I do drive it daily, it just has no power due to the stumbling. I have noticed that when it gets up to about 25mph, it seems to run better. Not good, just better.

That's about where I stand right now. Suggestions/comments are greatly appreciated. I've learned so much working on this car, I enjoy learning more. Even if you "assume" i've checked it, ask. I may have over looked something or over thought something.

Thanks in advance,
Jason

 

Just a few random points, but your symptoms sound electrical rather than fuel, to me:
Is the new ignition coil you used a correct low impedance one?
Did you gap the plugs to 25 thou?
Have you checked (this is a point from the Great XJS prophet Grant Francis) that the shielded wire from the Lucas ignition amp - the one you shoved the new HEI module into - to the ECU is OK for the first foot or two?
I suggest you test this and test the TPS at the ECU plug as well as checking the wiring and voltage at the engine end. I put a scrrew into the bulkhead during my rebuild and it produced exactly these symptoms by just half-shorting the ECU loom
Also there are resistors in the Lucas module (separate from the HEI bit) that go bad, these can be bypassed, but someone else will have to explain how! Swapping the Lucas amp for a known good one is not a bad idea if you can get hold of one.
Are you SURE the wires from the bottom of the dizzy to the Lucas module are OK? maybe when you redid the pickup these got damaged/somehow broken?
Are you SURE the Mustang TPS is giving the correct signal to the ECU (this is my favourite bet on the cause)?
Have you cleaned up the connector that joins the injector loom to the module on the inner wing US passenger side?
Have you tested that the injector loom is OK?

The 16CU is a direct swap and is a better made unit than the 6 CU, and your car, as an '86 should have had the 16 as standard. I doubt it is the ECU, more likely the newer one is more sensitive to the actual problem.

Good luck, you will find it, Courage as my French neighbours say.
Greg

 

Thanks for the reply, Greg.
Here goes:
New coil is the DAC... something that was a suggested replacement. (Prenco 36-1191)(UF241) I think i measured the Primary at .8ohms
Yes, i gapped to 25
i checked continuity of the shielded wire that reports to the ECU, i will check it again, and check for it being grounded.
I checked the TPS at the TPS connector, but will check at the ECU.
I checked the resistors inside the ICM. They read what they are supposed to be. I have a junkyard ICM that i can try.
I checked the wires from the pickup coil to the ICM both for continuity and signal. (AC signal when cranking). even borrowed an O-Scope from work to look at the wave form. looked fine.
Injector loom to the module on the inner wing? if that's the power resistors, yes.
I used a noid light to verify all injectors are firing. but i do think something is telling them to stop firing which causes the stumbling.

I'll leave the "new" ECU in there and keep trying things.

-Jason

 

update:
On a whim, i replaced the ICM with one i had snagged from a junkyard. This was after i checked the shielded wire for continuity, ground, and connection on both ends. Took it for a couple short test drives and it all seems sorted. This is after i've replaced the HEI module inside the ICM that i have 3 times. **KNOCK ON WOOD** I'm going to keep driving it and fixing the random other things that are not quite right, but, for the moment, the major problem of it running seems to be sorted.

Thanks to all who replied and helped out. The only thing i can guess was happening was that the tach signal was getting dropped by the ICM somehow at random times which would cut out the FI signal causing a stumble. I never checked the tach signal going to the ECU since i wasnt sure what it was supposed to look like. I guess that's what you'd call a half fail.

thanks again all,
Jason

 

Congrats. If it stays OK, then save up and buy and fit a new OEM amp, put the current one in the boot, and sleep easily! And you have done lots of preventative stuff along the way, so nothing wasted, that's for sure.

Greg

 

Good work, Jason.

Fifteen months ago my XJR was beating me up....long story.... and I fixed it 'on a whim'.

Sometimes things just work out

Cheers
DD