Hi all,
I have been troubleshooting my recently acquired 1989 XJ-S. I bought it from an insurance auction as it was identical, bar the colour, to my other car I’ve had for some time, thinking it would be great for parts. It was written off for flood damage, I’ve since established the water level was halfway up the tachometer.
When I got the car, I figured I might as well see if I could get it to run more for interest than anything else, and bar the starter motor which I replaced with a spare I already had, I got it running and driving- everything works. Of course, this was after I spent some days dismantling just about everything electronic (popping caps off every relay), drying them out, and spraying them with a healthy dose of CRC 556, a water displacing penetrating oil designed for this purpose. Also changing all the fluids etc. Even the stereo and convertible top works! Hence I think the car deserves to live and is too good for parts-the body has no rust, and I mean no rust, and looks to have been in better nick than my other car pre flood.
It starts and idles okay, though a slight random misfire has developed at idle and above half throttle it runs like absolute garbage. It doesn’t matter whether you’re in park or driving, if you gradually push on the throttle, the revs build (with a very slight miss) until it reaches a point where the engine cuts out randomly sometimes for up to a second, with periods of bursts of running and significant back firing out the exhaust.
Driving it creates a sensation of surging. The backfiring to me suggested ignition fault, but Initially I swapped out the 16CU fuel injection ECU for one from the other car which I know to work, which fixed it for one drive, but the next time I started the car it was back to the same. I think this was coincidence rather than fixing it. The Marelli control computer was still quite soggy and corroded when I got the car, so I didn’t trust it, so I replaced it with a brand new one I bought NOS, which again I tested in the other car- perfect. I tried another set of coils, no difference. The throttle position sensor on the underside of the pedestal seems to work and when you put the meter across it the resistance changes smoothly as you would expect. Someone may advise me that I’m not testing it properly. The inside of the cap I’ve cleaned out and seems to be in good condition, as does the rotor.
The tacho does not drop to zero when the engine quits, and behaves as you would expect so if that is fed from the crankshaft position sensor, then that must be working. I don’t understand what the flywheel engine speed sensor does, and whether it would create this condition. When above half throttle, both A and B bank behave the same. At idle, and lower throttle positions, both misfire occasionally, though the left (B) bank is slightly worse. So I think the issue is something common to both banks. The injectors seem to fire continuously and the backfiring suggests to me that the fuel ECU is still getting engine speed signal and operating correctly. Changing the map with the red jumper makes no difference to the misfire.
I’ve racked my brain trying to think what could cause this weird behaviour, if it dropped onto six or quit completely it would be easier to diagnose.
Problem is, I can’t trust anything as it’s all been underwater. Does anyone have in suggestions as to what could cause this? I can upload a video or something showing the running if it helps. What are the best next steps? How can I properly test sensors? I have another identical car so I can swap components to diagnose if needed. I’ve read lots of other topics though haven’t found anything yet comparable to this situation.
Thanks in advance, Ben.