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Renix - Random Starting Problem

T

TickDontPick

NAXJA Forum User
Location
Arizona
I have an '89 XJ Laredo 4.0 AW4 NP242....

I got in her today to go to school, and she wouldn't start. I just got home, and she fired up 1st try.

How should I go about diagnosing this? I dont have any diagnostics tools, but I'm willing to buy some as I'm sure I could use it later. Im just not sure where to start. Should i get a multimeter & test electronics? I have a feeling its not a fuel problem, but then again I havn't looked into it. Any insight would be nice, as I don't really know what I'm doing, but I am a quick learner. My Haynes repair manual wasn't much help, but maybe it'll come to use when I identify the problem. Thanks

Joel
 
I can hear the fuel pump delivering fuel when I turn it to the ON position. It would crank, but it wouldn't start. Im not sure what other information to give. I just replaced the IAC and fuel filter, tuned up less than 3k miles ago, oil change less that 1k mile ago.
 
Since it's cranking, your ignition switch and starter relay are probably good (those are the first things you check when it does NOT crank!)

If the fuel pump is running, then its relay and the ballast resistor are just fine.

Have you tried disconnecting/reconnecting the CPS connector yet? It's near the rear end of the fuel rail, a 3-cavity flat Weatherpack. If that works, then take the connection apart and clean it. It might also want replacing outright - you don't mention having replaced it, and they have a useful life of ~150-180Kmiles.

Why do I bring that up? The CPS signal is used to time both fuel injection and ignition. The RENIX system will "guess" until it gets a hit if the SYNC sensor (the one in the distributor) is out, but it don't do nothing if the CPS is shot. Having run into that before, I can attest to it. I typically keep a couple spare CPS units around, since I have four XJ's here.

Dealership probably wants somewhere around $40-50 for the thing, you can find it for a bit less online. You'll want one out of a 1989 or 1990 - 1987-1988 units are a little different, and the 1991-up is a different type of sensor entirely.

5-90
 
Ya know, I have had some previous problems where people have told me it could be the CPS, so I think I may just go ahead and replace that. I was having a starting issue where the car would fire, but idle would jump up & down from 0-500RPM, and the whole truck would be shaking. Almost like it would be stalling over and over again without me doing anything. Some carb cleaner in the TB seemed to fix that for now, but ive also had problems where I would let off the gas and the idle would drop significantly lower than normal, and then jump back up.

So could these be CPS related also? I have 123,XXX miles, so I might head to checker to see if they have one. Is it a dealer part it could I get one there???
 
Iffy. Best to go to the dealer, and get OEM. I've had very spotty luck finding them in the aftermarket.

123Kmiles? Hell man - you're not even broken in yet! ALL FOUR of my rigs have more than that, and at least two drives from SJC to IND!

If you're having trouble with "wandering idle," check for vacuum leaks as well. You can get replacement Nylon lines from NAPA (they're probably due...) and I have just gotten in the habit of retorquing all my manifold bolts once a year - I usually catch one or two loose. That's not the fault of the bolts, but a design thing - the inline six presents a long sealing surface, and the differential expansion of the aluminum intake and steel exhaust flange (aluminum expands twice as fast as iron) will work to "unscrew" the bolts over time. Be sure to check around the base of the EGR valve - I've had that gasket burn through and cause me trouble a few times...

5-90
 
What year? If you're RENIX (1990 and earlier,) then it's normal - it's the computer looking for a valid CPS and SYNC signal. If it takes longer than usual to start, and it sounds like the timing's off, you've lost the SYNC sensor. If it doens't try at all, you've probably lost the CPS. RENIX will work without the SYNC - just not well. It will not work at all without the CPS.

5-90
 
5-90:

Im still not exactly sure where to find the Crank Position Sensor. From your description it sounds like it is under the hood just in front of the throttle body, but In my Haynes guide it says that it is mounted on the tranny housing. Is there two CPS's by any chance? Like maybe a camshaft position sensor?
 
The "Cam Position Sensor" is what I refer to as the SYNC, and it's in the distributor under the rotor.

The CPS proper is at 1100 on the bellhousing (looking forward, along the axis of the crankshaft,) but the connector (the part you want to try unplugging and replugging) is near the rear of the fuel rail up top - that 3-pole Weatherpack I mentioned.

If you look while you're back there, you should just see the CPS on the bellhousing - it's retained by two bolts (7/16" or 11m/m heads - the same thing either way) and those bolts are FAR easier to hit from underneath with about two feet of extensions and a socket with an inbuilt universal joint, believe me!

DO NOT drop either of those bolts down the hole into the bellhousing, or it's even odds you'll be pulling the transmission to get them back! They're also a funky variation on the "shoulder bolt" - you can't get them just anywhere - I've tried. I'm thinking it's CNC time for replacements - with a couple of improvements built in. What did I do with those spares I have?

A handy trick for getting those bolts in and out easily - chew some gum while you're getting set up. Push the wad of gum into the socket, and it should help hold the bolts. Put the socket in the freezer overnight to freeze the gum, and it's a lot easier to pull out later. One of the "improvements" I've got in mind will be an O-ring groove around the head to make the damn thing stay in the socket on its own...

5-90
 
5-90 said:
The "Cam Position Sensor" is what I refer to as the SYNC, and it's in the distributor under the rotor.

The CPS proper is at 1100 on the bellhousing (looking forward, along the axis of the crankshaft,) but the connector (the part you want to try unplugging and replugging) is near the rear of the fuel rail up top - that 3-pole Weatherpack I mentioned.

If you look while you're back there, you should just see the CPS on the bellhousing - it's retained by two bolts (7/16" or 11m/m heads - the same thing either way) and those bolts are FAR easier to hit from underneath with about two feet of extensions and a socket with an inbuilt universal joint, believe me!

DO NOT drop either of those bolts down the hole into the bellhousing, or it's even odds you'll be pulling the transmission to get them back! They're also a funky variation on the "shoulder bolt" - you can't get them just anywhere - I've tried. I'm thinking it's CNC time for replacements - with a couple of improvements built in. What did I do with those spares I have?

A handy trick for getting those bolts in and out easily - chew some gum while you're getting set up. Push the wad of gum into the socket, and it should help hold the bolts. Put the socket in the freezer overnight to freeze the gum, and it's a lot easier to pull out later. One of the "improvements" I've got in mind will be an O-ring groove around the head to make the damn thing stay in the socket on its own...

5-90
Click to expand...

I wonder if just shoving a rag or plastic bag into the hole around the CPS entry into the bell housing would not solve the potential problem of dropping a bolt into the housing? Does the grommet keep the bolts from falling into the housing?
 
Ecomike said:
I wonder if just shoving a rag or plastic bag into the hole around the CPS entry into the bell housing would not solve the potential problem of dropping a bolt into the housing? Does the grommet keep the bolts from falling into the housing?
Click to expand...

Dunno - probably, but I didn't even see a grommet until my fourth XJ.

That's why I "slot" the bracket the first time I replace the thing - once I do that, I no longer have to remove the screws to change it, merely loosen them. The sensor can be finagled into place from above and weighted down (I have a two-foot-long screwdriver that works handily for that,) and then roll under and tighten up the screws. Et viola! CKP replaced!

I'd knocked it down to about a fifteen-minute job on my personal rigs. I may take my universal socket and cement a magnet into the base just for working on other people's rigs, just to be safe...
 
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