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Embarrassing Engine Starts

P

PuddinHead

NAXJA Forum User
Location
Virginia
The stats 90 Renix 4.0 with aw4 tranny and 170k miles. This is more embarrassing at this stage, but I don't want to get caught out and not able to limp home. Intermittant Prob: Engine starts but idles very bad, shut off and restart. Engine runs better, but still rough, shut off and restart. Engine runs perfect. If you try to drive with the engine running rough it does not smooth out. I'm thinking IAC and cleaned the outside of the the 'bullet' but don't if anything else can be done. I can't find any threads with same symptoms. I know it could be the sync pulse gen in the dist. How do you change it out? Any suggestions (about my jeep prob:laugh3: ) appreciated.

--Puddinhead
 
The SYNC sensor in the distributor requires actually pulling the dizzy to change it - the shaft also needs to be removed.

Try this first - take one of your booster cables, clamp one cable to the engine block (somewhere clean,) and clamp the other end of the same cable to the negative battery terminal. If that cleans things up, you've got a faulty ground (very common on RENIX) and you should clean them.

If that doesn't work, try moving the clamp on the engine to somewhere clean on the chassis - same problem, different location. The chassis on RENIX grounds by way of the engine block, and the ground is the "zero reference" for the various sensors and the ECU - so a good chassis ground is important.
 
Cleaned grounds, especially motor block ground. Replaced both battery cables and ran an extra ground inside cab. Still acting up. Only other thing is some running lights I wired up (w/relay and fused) that come on when key is turned on. This is an extra load for battery when turning over engine but battery is only 6 months old. Could it be a flaky MAP sensor or CPS? Could it be in the ignition switch?
 
I have the same exact EXACT problem with my 88... All grounds are zero ohms, and I've done everything from complete tune up on. I was thinking a sensor, but aren't the sensors out of the loop until she reaches operating temp?

This is such a common problem, and the only answers I have ever gotten were guesses about the grounds, or O2 sensor, or TPS (which in my case, TPS is new and adjusted properly and tested by me)

Can someone who has defeated this gremlin please share?

Injectors were new before I got it, and it was not doing this when I got it.
 
Sounds more like an intermitent electrical contact. Could be anywhere, relay, wires (broken but making physical contact by arcing), sensor connectors, battery, or battery clamps, alternator.....

I would get under there with a volt meter and take readings and wiggle wires while it is idling rough!!!!
 
Last edited:
Ecomike said:
Sounds more like an intermitent electrical contact. Could be anywhere, relay, wires (broken but making physical contact by arcing), sensor connectors, battery, or battery clamps, alternator.....

I would get under there with a volt meter and take readings and wiggle wires while it is idling rough!!!!
Click to expand...
No, this is ONLY after a cold start. It stalls once or twice, then runs perfect as soon as it warms up. EVERY TIME, it is not intermittent. (not in my case at least, and the op states that it runs perfect after restarting twice too)
 
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xj88superjeep said:
No, this is ONLY after a cold start. It stalls once or twice, then runs perfect as soon as it warms up. EVERY TIME, it is not intermittent. (not in my case at least, and the op states that it runs perfect after restarting twice too)
Click to expand...
:twak: Uh, anything else you forgot to mention besides it only doing it on COLD starts? LOL!

Still could be bad wiring, shorted or damaged wire, bad contacts, as they can be temperature sensitive, but it could also be the CTS or MAT (also called the IAT) temperature sensors. When my MAT sensor went bad it would not start when cold (cold mornings), only started when hot late after noon when it was hot outside.

The CTS and MAT resistance can be easily checked against factory specs with an ohm meter.

Search "Renix or RenX Temperature sensor" here for the correct CTS and MAT resistance values, they are the same, and posted in about 20 threads in this forum.
 
You say it does not go away when the engine heats up during the first start, but goes away after 2 starts, so based on that and it being only cold starts with the problem, maybe look at the CTS and MAT wiring harness connections themselves as well, and the starter cables including the starter ground to the engine near the dipstick, both the wire and the surface contacts. A lot amps going through there at start up, that can clean a poor loose contact between start ups, and then cool, loosen, and reoxidise overnight.
 
Ecomike said:
You say it does not go away when the engine heats up during the first start, but goes away after 2 starts, so based on that and it being only cold starts with the problem, maybe look at the CTS and MAT wiring harness connections themselves as well, and the starter cables including the starter ground to the engine near the dipstick, both the wire and the surface contacts. A lot amps going through there at start up, that can clean a poor loose contact between start ups, and then cool, loosen, and reoxidise overnight.
Click to expand...

My problem can happen on warm startups. I took the cables loose at the block ground and cleaned them with brake cleaner. I think there were three other grounds there besides the battery neg. Both battery cables are new. I replaced all of the relays on the pass fenderwell today. And I plan on rigging a time delay for my daytime running lights relay so i have more voltage at startup. The only other thing is my cruise doesn't work so maybe I have a bad place in the wiring somewhere. Life is good.:dunno:

/BTW I removed/cleaned and ohmed the MAT sensor, it was okay according my table. If the CTS goes sour the RENIX ECU will ignore it IIRC.

FRIDAY I'm off to the salvage yard. This guy has about a dozen XJ's and I found one today with MY name on it!
 
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PuddinHead said:
If the CTS goes sour the RENIX ECU will ignore it IIRC.
Click to expand...

True, unless it is just lying to the ECU and telling it is 200 F when it it is really 50 F, then the ECU makes bad desicions based on bad data. Mine had no CTS connected the first year I owned. Didn't even know it was there. PO had disconnected it for some odd reason. THe ECU can ignor no sensor, or infinite resistance, but not a reasonable, in the normal range lie from the CTS!
 
Ecomike said:
True, unless it is just lying to the ECU and telling it is 200 F when it it is really 50 F, then the ECU makes bad desicions based on bad data. Mine had no CTS connected the first year I owned. Didn't even know it was there. PO had disconnected it for some odd reason. THe ECU can ignor no sensor, or infinite resistance, but not a reasonable, in the normal range lie from the CTS!
Click to expand...

OH KAY, I'll ohm the CTS. I figure it's got a bunch of gunk on the probe side. I guess I can stick my finger in the hole to keep the antifreeze from pouring out....
 
OK, Mine is a lot more like: Cold start, starts perfect, put it in gear, it stalls. restart, starts perfect, put it in gear, runs until I let off the gas to stop when I back out of the driveway, it stalls again. restart, starts perfect, and it runs great the rest of the day unless it sits for 2 hours or more. If I shut her off to go into a store or something, it NEVER does it. The longer it sits and cools off, the more severe the problem is.
 
PuddinHead said:
OH KAY, I'll ohm the CTS. I figure it's got a bunch of gunk on the probe side. I guess I can stick my finger in the hole to keep the antifreeze from pouring out....
Click to expand...

No need to pull it, just disconnect the harness connection and check across the 2 wires on the sensor at room temperature.
 
xj88superjeep said:
OK, Mine is a lot more like: Cold start, starts perfect, put it in gear, it stalls. restart, starts perfect, put it in gear, runs until I let off the gas to stop when I back out of the driveway, it stalls again. restart, starts perfect, and it runs great the rest of the day unless it sits for 2 hours or more. If I shut her off to go into a store or something, it NEVER does it. The longer it sits and cools off, the more severe the problem is.
Click to expand...

Sounds a little like a sticking IAC.
 
Ecomike said:
No need to pull it, just disconnect the harness connection and check across the 2 wires on the sensor at room temperature.
Click to expand...


I was just thinking ahead... When I tested the MAT it was out of spec, so I pulled it and the sensor end was gunked up. Cleaned it and it checked okay. Figured history would repeat itself with the CTS. It always does.:banghead:
/
 
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