My 88 XJs been running rough so I packed Cruiser54's list into the parts-canon and ****ing sent it bud. 🤣Wiring harness gone over, almost every sensor replaced. Plugs, wires, rotor, coil, injectors..... the kitchen sink.
Weak spark and voltage at injectors really low. (I know, I know "testing voltage at injectors produces inconsistent readings because 'blah blah blah'...." It consistently read 2.something max on all injectors.)
New TPS reading 2.54 and 0.20 at max adjustment.
Harness disassembled completely including deletion of "that one plug" on the driver's side, additional ground etc.
"Computer" is sus.
Grounds tested at ECM. 12.1v in, grounds @ 0.0 ohm. Out to TPS should be 5v+/- it's 2.55v
Visual inspection of circuit board revealed no visible abnormalities.
Anybody in NorCal have a 1990 Renix brain they are willing to donate to the cause? I mean seriously.... **** this thing.
i bought an ecu brain for my 1990 on ebay, about 125 bucks delivered. I see many for sale. probably worth having a spare, even if your existing brain is good.
Also do NOT forget to run a manifold vacuum gage test. ( this is what the MPS reads) I went after all kinds of injectiin and ignition dead ends on my jeep, then I ran a manifold vacuum gage, and found the engine had horrible vacuum. Root cause was the egr valve seat had come apart such that the egr was always flowing, even when closed! this screwed up the MPS signal, and the intake oxygen level. The egr appeared to operate, you could see the plunger move in and out, but till you removed the egr from the manifold you could not see that the valve seat was gone! The vacuum test told me I had very low vacuum, so I thlught I had an air leak (as timing was good, and compression was known good, and I had a fresh exhaust system (i.e. no clogged cat nor blocked tail pipe) I checked for intake leaks with aerosol carb cleaner, I blocked off the brake booster hose, trying to find the source of the leak that the vacuum gage proved existed. Finally I borrowed a smoke machine, and smoked the intake system from the throttle body. smoke poured out the tail pipe! now an open intake and exhaust valve if overlapped can cause this too, so I shoved a rag in the tailpipe to seal it
so as to build pressure in the manifold, it was then I saw faint wisps for smoke from the egr valve stem. Pulled the egr and flund the valve set inskde, a press fit into the body had come out. Blanked off the egr with a steel plate and she then ran great, and the manifold vacuum gage test was normal. Then bought a new egr (which are getting rare for classic vehicles.)
So run the test, see if that part of the system is healthy or not, it takes a $30 gage, and five minutes. If bad, then there just a few things to look at, so it really can help narrow down your hunt for the problem.
My egr fail caused horrible running issue, hot, hunting idle, hard starting, poor power. and it was not consistant, the valve seat would rattle around, some times landing in the correct spot, thus a tiny leak, other times dancing out completely, or even getting wedged. And the leak when big was huge, much larger opening than the egr should allow even if wide open.
So run a vacuum gage test, before doing much more. if you got weak vacuum, then you MUST address that, and it is not an injection issue.
If you dont have good vacuum, it will never run correct. Test it! A five minute test is all it takes!
Also, the Snap On MT2500 is a great tool, (link below) it is a scan tool, and allows you to read what the brain reads, vacuum, O2 sensor, temps, injector pulse, timing, knock sensor, etc... It plugs into the factory diagnostic connectors near the engine bay relay panel on Renix jeeps. I got mine on craigslist, ebay has a lot of them too. The tool, it will even read manifold vacuum via the MPS. good diagnostic tool ( but it does not replace an analog manifold vacuum gage, you need the analog vacuum gage to be able to see fine fluctuations in vacuum that a digital gage cant display)
Cruiser, the Patron saint of XJs. Respect and thank you for the wealth of knowledge you compiled and shared with us. You are a legend sir.
Rob, thank you for such an in-depth and thought provoking reply. I will be picking up a vacuum guage ASAP.
I'm relatively certain that's not the problem as I tested the diaphragm and the gasket is in great shape. However as I said, I'll be picking up a guage and testing it that way as well.
Quick question though.... Orange wire with black stripe is not supposed to ground right?
On the Renix, the ECU controls the +12 side of the injectors and they share a common ground. So you're not going to see 12 volts at the injector with a meter. Does the +5-volts to the TPS recover if you start unplugging other sensors like the MAP?
lawsoncl, no. I just disconnected every sensor possible and every remaining coupling in the engine bay with no effect on the voltage.
I see where you're going with that though and will continue to monitor the voltage as I disconnect things chasing this orange wire short.
Yesterday I knoticed the breaker for the power locks and seats(I think) on the fuze panel was hot so there's another short I gotta chase. Pulled it for now😒
Oh.... And my light switch turns on my stereo... Even with the stereo fuze pulled from the panel...🤣FML🤣
Quick update.
While tearing apart the dash searching for the short between my stereo and headlight switch, I decided to delete the power/comfort switch.
*snip-snip-splice* jobs done........
y'ever get that feeling? Trust it. Mine said chase that striped wire. Black with a yellow stripe.
A quick peek at the end confirmed it, see pictures.
Seems the mechanic that welded my "frame/stiffener" ****ed up more than just pulling my spedo cable out of the transmission 🤬
So, at least I have an understanding of who caused the problems.
Transmission wiring harness is now SUS and must be thoroughly inspected/tested...FML.
quick update
after chasing every available wire to no avail I turned to the last bastion the short could be hiding in.... the dash.
oh, c'mon that looks like fun right? Right?😬
.............
I've been fighting with this thing for about a year and a half...
I've tried everything...
I gave her both barrels of the parts cannon.....
I [expletive deleted] checked every [expletive deleted] wire....
This morning, while putting the dash wiring back together, I gave the tps a test. Its read 2.54 for so long I about wept. I'm not 100% what the problem was, but I'll update as soon as I figure it out.
I actually took the whole wiring harness out of a "running" parts-jeep (89) and the harness out of my 88, laid them both out on the ground next to each other and tested/repaired/replaced every single wire to create a hybrid 88-89 harness. (Engine bay wires are 89 because factory delete of 101 connector and 88 was faulty. Everything aft of firewall is 88 because "limited" model options not included in 89 harness)
Every wire has been scrutinized. Every connection tested.
My low voltage was caused by something in the running lights/light switch/stereo or climate control wiring under the dash. 🤷🏽♂️
So I'm thinking my electrical system should be fine. Fire it up and its running better but still like crap. Like its firing late.... sputtering and backfiring both out of the intake and exhaust.
Thinking timing chain? So I get one, replace the gears and chain as per instructions. (Old ones where kinda worn, kinda loose.... not bad-bad though.) Put it all back together and no change.
dug out old CPS and advanced it, engine runs better.... but.
Still not firing right... like its firing while the exhaust valves are partially open.
Just before writing this, I fired it up. It sounded ok through the initial rpm spike but when it idled down it coughed and sputtered a couple times before mildly backfiring... then it ran backwards for a few seconds.
Kinda at a loss
salute
