Renix issue (hot restart, closed loop)

1988 XJ 4.0 Auto

New TPS (adjusted properly)
New IAC (fixed surge)
Cleaned TB
Cleaned C101
Fixed 5V ground
Tested CTS (seems to be working at 70* and 100*)
Tested/Cleaned IAT (reads correctly)
Disconnected vacuum source to EGR at manifold
bypassed ballast resistor


Jeep starts and runs prefect. Wont restart (just cranks) after hot, requires a squirt of ether to get it going. Once its started it idles at 3k rpms. If I dont ether it, it will just crank and crank until cooled down.

If I start the Jeep and let it idle it runs great cold, as its heating up and I am hanging around it...I can restart it just fine until I hear it go into what I think is closed loop. When closed loop happens it wont restart or if it does it will have the high idle or crank no start condition. This is my indicator when the problem hits.


I am noticing black out of the tail pipe also. I attempted to check the O2, but couldn't find proper info on it. I am also not sure if an o2 will cause a Crank No Start.

Not certain these are your issues, but where I'd look first. A 3000 RPM idle is a serious vacuum leak or more likely a TPS issue. People get fixated on components and forget about the connectors and wiring. Check your manifold bolts. Make sure the EGR vacuum line you disconnected is plugged off at th manifold. Make sure the EGR is closed, if it sticks open low RPM operation is going to be really bad.

Hot no start is most likely a CPS issue. I've had to pour a pint of water onto my CPS to cool it enough for a start. Cruiser has some good write ups on CPS testing, test cold and hot. EcoMike has also done numerous posts on CPS.

If it is an IAC issue try starting while giving a little gas pedal, maybe 1/8th. Low voltage, a battery issue or an alternator issue can cause iffy IAC function. I've had my IAC be the first indicator of a voltage issue, before I even noticed a weak battery or low alternator charging.

Check for weak spark, not likely to cause a high idle but can cause starting problems. Weak spark usually causes a very bad idle. It can be part of a CPS issue, spark can get sporadic.

You may have multiple issues.

Do a search on heat soak.

What Old Man posted got me to thinking. The reason heat soak is less of an issue with the Renix and early model XJ's with a fuel return line is, the fuel circulating during cranking cools the fuel rail down. Heat soak is much more of an issue with the later model XJ's without the fuel return.

Maybe the "B" pin on your starter relay isn't working. And/or the CPS pulse isn't regular or strong enough to close the fuel pump relay during cranking?

My 87 has been doing this.

For a while, unplugging and plugging back in the CPS seemed to work, Then, the water poured on the CPS did. 2 days ago, only waiting a long time worked. I put a new CPS on it but haven't had a chance to verify. My fuel pressure was good every time.

I'm curious if you have weak spark and that's why ether will light off and not gasoline.....

I'm carrying a good used ICM/coil with me. LOL.

maybe2fast said:

1988 XJ 4.0 Auto

New TPS (adjusted properly)
New IAC (fixed surge)
Cleaned TB
Cleaned C101
Fixed 5V ground What is a 5 volt ground?
Tested CTS (seems to be working at 70* and 100*)
Tested/Cleaned IAT (reads correctly)
Disconnected vacuum source to EGR at manifold
bypassed ballast resistor


Jeep starts and runs prefect. Wont restart (just cranks) after hot, requires a squirt of ether to get it going. Once its started it idles at 3k rpms. If I dont ether it, it will just crank and crank until cooled down.

If I start the Jeep and let it idle it runs great cold, as its heating up and I am hanging around it...I can restart it just fine until I hear it go into what I think is closed loop. When closed loop happens it wont restart or if it does it will have the high idle or crank no start condition. This is my indicator when the problem hits.

Closed loop happens in seconds after the engine starts, your problems are not with the O2 sensor circuit or sensors.


I am noticing black out of the tail pipe also. It is running too rich.

I attempted to check the O2, but couldn't find proper info on it. I am also not sure if an o2 will cause a Crank No Start.

Click to expand...

It will not cause your no start problem. The O2 system, many parts, grounds, power from the O2 sensor heater relay, relay, signal from the ECU to the sensor and then back, wiring, connections. There is thread here from years ago I wrote "Testing O2 sensors" Google it with Naxja in the search string.

The two problems may be related or not. You need to monitor the fuel pressure all the time and see if it is 29 to 39 PSI during acceleration and deceleration and idle. Check the vacuum line to the fuel pressure regulator on the fuel rail front side to see if it smells anything like leaking gas from the FPR diaphragm. If you smell fuel the FPR is bad and leaking fuel past the ECU's control path and flooding the engine. Or if the cranking and running pressure is not between 29 and 39 psi then the fuel pump is bad or tubing in the gas tank from the pump to the flange has a leak. Fuel pressure below 29 psi on a hot engine can cause vapor lock.

Did you close off the end of the vacuum hose you disconnected from the EGR???? That could be an idle speed issue too.

Last time I had your problem it was a bad IAT temp sensor, but you say you have tested it? All the sensor ground wires under 1 ohm to the battery negative post with engine and power off?

cruiser54 said:

My 87 has been doing this.

For a while, unplugging and plugging back in the CPS seemed to work, Then, the water poured on the CPS did. 2 days ago, only waiting a long time worked. I put a new CPS on it but haven't had a chance to verify. My fuel pressure was good every time.

I'm curious if you have weak spark and that's why ether will light off and not gasoline.....

I'm carrying a good used ICM/coil with me. LOL.

Click to expand...

Bad ICM was the only one I never could test to verify, but the burned and scorched potting material over the circuit board was a great clue as I swapped it out, but it caused a no start under any condition issue on mine. It did start out as a sometimes it would start issue that had no other causes.

BTW, on another topic, we finally got the 87 gas tank flange loose using a BFH and scrap metal 3/8" od by 12" long piece of scrap aluminum (spark proof). Been raining for weeks and have not had a chance to swap the pump yet and reinstall and test your tool again. It would not work for the removal.

My first thought on this "Wont restart (just cranks) after hot, requires a squirt of ether to get it going." is that is not a spark problem, but a fuel problem. Thus my comments about testing for possible lock.

Damaged or crushed return fuel line???? Pressure too high and making it run rich??? and letting the fuel rail get too hot??

maybe2fast said:

1988 XJ 4.0 Auto

New TPS (adjusted properly)
New IAC (fixed surge)
Cleaned TB
Cleaned C101
Fixed 5V ground
Tested CTS (seems to be working at 70* and 100*)
Tested/Cleaned IAT (reads correctly)
Disconnected vacuum source to EGR at manifold
bypassed ballast resistor


Jeep starts and runs prefect. Wont restart (just cranks) after hot, requires a squirt of ether to get it going. Once its started it idles at 3k rpms. If I dont ether it, it will just crank and crank until cooled down.

If I start the Jeep and let it idle it runs great cold, as its heating up and I am hanging around it...I can restart it just fine until I hear it go into what I think is closed loop. When closed loop happens it wont restart or if it does it will have the high idle or crank no start condition. This is my indicator when the problem hits.


I am noticing black out of the tail pipe also. I attempted to check the O2, but couldn't find proper info on it. I am also not sure if an o2 will cause a Crank No Start.

Click to expand...

How to test O2 sensors on renix jeep

http://www.naxja.org/forum/showthread.php?t=1012701&highlight=oxygen+sensor+test

As for richness, the MAP sensor line between throttle body and it needs to be perfect!!

http://cruiser54.com/?p=280

alittle update, CPS AC voltage is .26V (.5V-1.0V spec). Replaced the CPS and had no change on the AC voltage. I found the CPS update harness in the Jeep! Replaced the harness and had no change on the AC voltage.

Started the Jeep and 3k rpm idle right away...strange because it usually has to be warm to do this. This lead me to believe since the CPS wasn't heat soaking (new,cold) it wasn't the issue. I unplugged the TPS and the idle lowered.

I checked the TPS many times but it seemed that the heat was effecting it. I checked my output voltage while the TPS was unplugged and 5V! Seems that the TPS was shorting when hot! I hope this is the total problem, and its solved.

I tightened the top intake bolts but they were not bad.

Fuel pressure was great, 30-40psi

So, a new TPS installed and adjusted properly solved the issue?

You've been using the method in my Tips for adjusting it, right?

As written this does not make any sense?

"I checked my output voltage while the TPS was unplugged and 5V! Seems that the TPS was shorting when hot! I hope this is the total problem, and its solved."

The TPS has no power or voltage when disconnected.

Did you move the CPS closer to the flex plate? What brand CPS did you use? Did you try a Mopar?

Ecomike said:

As written this does not make any sense?

"I checked my output voltage while the TPS was unplugged and 5V! Seems that the TPS was shorting when hot! I hope this is the total problem, and its solved."

The TPS has no power or voltage when disconnected.

Did you move the CPS closer to the flex plate? What brand CPS did you use? Did you try a Mopar?

Click to expand...

This was a typo. It was connected.

It was a RENIX sensor, both had similar resistance and exact output voltages.

8Mud said:

What Old Man posted got me to thinking. The reason heat soak is less of an issue with the Renix and early model XJ's with a fuel return line is, the fuel circulating during cranking cools the fuel rail down. Heat soak is much more of an issue with the later model XJ's without the fuel return.

Maybe the "B" pin on your starter relay isn't working. And/or the CPS pulse isn't regular or strong enough to close the fuel pump relay during cranking?

Click to expand...

You are somewhat correct. Yes, Renix has less heat soak because the fuel rail stays cooler due to recycling fuel back to the tank, but when you shut the engine off, you can still get the residual engine heat causing starting problems. It can boil just sitting there because the fuel isn't circulating when the engine is off. It then takes a bit of having the fuel circulate to cool down the injector and cause the bubble to go away. That is why once it starts and you drive it, all is fine.

i suppose a failed tps in a way that the ecu sees the throttle as open all the way will put it into flood clear mode and not start. interesting failure.

90xj06 said:

i suppose a failed tps in a way that the ecu sees the throttle as open all the way will put it into flood clear mode and not start. interesting failure.

Click to expand...

A failed Renix TPS/wiring or sensor can cause all sorts of odd issues. Failure to brake (RPM too high at idle), shifting to 4th gear one gear at a time at 1200 rpm each time so that you hit 4th gear even at WOT at 20 mphs (ask me how I know) that mimics a worn out engine if you have no tach to notice what is going on and don't know how to make sense of it. Odd dangerous shifting (not the AW4s fault). engine speed wandering.

Flooding on start up attempts (I think) and whose know what else.

It was a very strange failure. Started cold everytime perfectly. Warm restart was where the issue showed up. The most interesting part of it was trying to diagnose it, you could stand there on the 1st start as it heated up. It would suddenly change tone and run slightly worse. Once this happened it would not restart. If it hadn't happened yet, it would restart fine. Once you got it running on the 2nd restart it would idle at 3k.

maybe2fast said:

It was a very strange failure. Started cold everytime perfectly. Warm restart was where the issue showed up. The most interesting part of it was trying to diagnose it, you could stand there on the 1st start as it heated up. It would suddenly change tone and run slightly worse. Once this happened it would not restart. If it hadn't happened yet, it would restart fine. Once you got it running on the 2nd restart it would idle at 3k.

Click to expand...

Have you done the sensor ground test?