nord

New member from Europe, first post.

Background: I have a 1990 Renix XJ 4.0. The car blew a headgasket because of a bad previous repair and has sat for some 8 years with the head off.

When I got it, I realized it's just not worth it fixing that half-dismantled engine that has sat full of water in all the erong places for years, so I pulled a H.O. engine out of a 1995 ZJ that ran great until the moment I started taking it all apart.

I cleaned the engine, replaced oil, gaskets, seals, thermostat, serp belt, spark plugs and rrlated tune-up stuff.

I swapped Renix flexplate over, Renix CTS that lives in the water jacket down below exhaust manifold. I've obviously used Renix manifolds and distributor.

Got it all hooked up, fresh antifreeze and radiators, new fuel filter and injector seals, blown out all possible crud from fuel lines, took the tank off, it was awful so I replaced it with a mint one, put a new fuel pump as well. Fuel pump ballast resistor bypassed for good measure, I don't care about the noise.

It starts beautifully and idles great. slight throttle makes it stumble, from circa 1300 up to 2100 rpm it wants to shake really bad and sounds as if it backfires into intake. When it gets over that, it stops backfiring and picks the revs up really good.

So, where I'm at:
- Fuel pressure good.
- Injectors work, connectors aren't missmatched, according to the wiring diagram for a 1990.
- IAT and ECT sensors consistent to each other, give about 1.5kOhm at ambient (cca 40°C, probably more because heat-soak.)
- Crank sensor puts out .40V AC while cranking, and the thing starts every time.
- TPS adjusted to 17% of reference voltage, as per manuals.
- Knock sensor broken off, probably years ago. Looking for a replacement, but I'm lead to believe thst no one noticed any difference in running with or without it.
- O2 sensor condition unknown. Plus currently running no exhaust past the bellhousing, open downpipe so to say.

Oh, and alternator gives out 11.8v!? It has been rebuilt prior to installing the engine, all the grounding points check out (albeit they seem inadequate to me) and I've gone over every connection in engine bay and at ECU and cleaned them really good. It has voltage at one of the wires at tha connector with ignition On. This seems odd to. Is there supposed to be an alternator dummy light in the cluster? I only see a voltmeter there, which works...

So. That's where I'm at right now. I know my way around obscure Bosch injection systems of that vintage, but this seems like I've hit a wall right now. All the ideas are greatly appreciated...

Regards!

SP1990

Alternator putting out 11.8 would be the starting point for me. Not sure how vital the 12-13 volts is for a renix, but I had a car that would exhibit same problems, ignition didn't have enough voltage to function correctly at high rpm.

nord

Yeah, I guess. Though all sensors that I've tested so far give out ok readings. I checked the MAP for leaks and for operation, and it seems fine, and gives out 1.6v at ldle.

I have no idea as to why is the alternator weirding out in the first place.

DFlintstone

Got fresh fuel?
Mine did that years ago before I joined the forum. One of the wires off the injector harness down to the lower sensors would short when it warmed up. (long story there). WHAT A PAIN FOR OVER A YEAR! Found it, fixed it YEA! Unfortunately I don't know which wire it was.

You might go ahead with an NTK 02 sensor and look really closely for any issue in those wires. Maybe see if you can jiggle them while its stumbling....

The same thing comes up now and then normally with out a solution, hope you can end this thread differently!

2drx4

As said, fix the alternator. BUT, it should run fine at only about 12V. A Renix will typically run down to 9V in my experience.

Check the TPS through the range of motion. They get cruddy and will give screwy readings depending on throttle setting.

Make sure the MAP tube isn't leaking.

Check for vacuum leaks between the intake manifold and head.

Clean and refresh the grounds for the engine harness too.

ajpulley

You will need an analog volt meter to test the throttle position sensor through its full range of motion.

You can do the same thing with the manifold absolute pressure sensor with a hand vacuum pump. If there is a crack in the hard plastic line leading to the MAP sensor, replace it- don't use rubber vacuum hose.

Imperfect grounds cause a lot of issues.

2drx4

No you don't. Get a digital meter that wasn't a freebie from Horror Fraught and it you can do it without trouble.

DFlintstone

Actually, it takes a pretty spendy digital meter to perform the sweep AJ is referring to, most will just display nonsense with a changing signal. Even a cheap analogue, on the other hand can show a "sweep".

ajpulley

The reason you use an analog meter is you want to see an analog voltage reference, i.e. not a digital numeric screen that has a sampling rate much, much faster than you can interpret the digits you see. The throttle position sensor is simply an adjustable resistor, or a potentiometer. As the resistance changes along the resistive strip that increases resistance from one end to the other, you can visualize this with a needle on a volt-ohm meter, mimicking an analog signal. The needle should move smoothly as the throttle position changes. Jerky movements, or the needle dropping suddenly along the path between closed and open throttle indicates the potentiometer has become carbonized, partially delaminated or otherwise has become defective beyond how it was intended to work and thus causes issues with the analytic voltage the ECU is recieving to interpret the throttle's position. I own a few rather pricey digital meters. Sometimes, there just isn't a substitute for an old fashioned VOM. Of course, an oscilloscope will work too, if you want to spend the money for one, and can find one.

I am a Master Electrician, a commercial and industrial electrical contractor with my own business, and I work with PLCs and control circuits a lot. I also hold an FCC license with a background in electronics. Once in a while, I know a thing or two.

nord

Guys, no need to fight. I've already checked TPS sweep when I was adjusting it, and it was fine. All the sensors and grounds are.

And there my problem was. I went chasing all the high-level things, and forgot about the simplest things. Apparently, yesterday I finally welded the exhaust so I could hear the engine over all that rumble. What sounded like somewhat normal running before - now sounded way off. Like it was running advanced... And there I was, staring in the distributor and wondering how did I miss it. The thing was geared one tooth off.

Realigned it and presto - it purrs like a 4.slow should.

Just wanted to give this thread some closure, in case anyone else has the same issue. If you've gone over everything and still nothing - check your distributor indexing again.

Funny thing. Thread was called Renix woes, and it turned out not to be Renix's fault. Poor thing compensated ignition angle as much as it could actually.

cruiser54

As I read through this thread, I was gonna suggest double checking the indexing. Great job!!

2drx4

I can do it fine with a $60 (Canadian dollars at that) meter. I guess that's too spendy for most people.

DFlintstone

10-4.I check Ebay now and then for a used Fluke or something, no luck there. Tried a couple $20 something meters a few years ago, no luck there ether. Getting buy with the cheapo Radio shack analogue for the rare sweep : )

Glad you both nailed it and got back to us Nord