Renix 4.0 Starting Issue

[jeepdude89]

I am having trouble with my 88 MJ starting lately. It'll crank over and start but immediately die. I can try again and it'll fire right up, but idle very high (about 2500rpm) for about 20 seconds and then idle down and run fine. I have done all of cruisers tips and have replaced the following-cap, wires, plugs, TPS, IAC and I have also cleaned the throttle body, replaced the gasket and repaired the cracked MAP sensor line. Anybody have ideas? 1988 MJ 4.0 5 speed

Thank you

[jeepdude89]

Got this figured out. Turned out to be a bad latch relay. Got it fixed and it starts right up on the first try now and idles like it should. Just wanted to let everyone know in case someone else runs into this.

[cruiser54]

Thanks!!!

 

This?

 

[jeepdude89]

You're welcome! Thank you for all your Renix info! It's a huge help!

 

Nope-this one-power latch relay

 

[cruiser54]

Interesting......

[jeepdude89]

Yeppers-sure is

 

Now to find a 89-90 ECM and get some more power out of it!

[cruiser54]

Yeppers-sure is

 

Now to find a 89-90 ECM and get some more power out of it!

Nice upgrade.

[Jeep Driver]

Found this elsewhere-

 

"The Renix ECU basically stops functioning when the Key is turned off... that's not a good thing because your XJ was just running with the idle stepper motor (ICS) set to idle a warm motor at 650rpm or whatever... 

"The idle air requirements for a warmed motor are no where near the air requirements to start a cold motor... so the B+ Latch relay was added to the system (by Renix in the level I implementation of this ECU line - never released in the US, but found in Renaults) . When you shut off your key, the B+ Latch relay continues to power the ECU so that the ICS backs-out and is ready to admit the additional air-flow required for your next start (time of the latch is a second or so and is 'controlled' by the ECU).

"Additionally, the B+ line (in the ECU) provides the ECU with a clean and seperate power source (since it passes though the relay and not all the way though a noisy switch) -- This is useful for items like injector and/or ignition drivers where there are very-square output waveforms that would ordinarily drive noise back through power supplies and into other more delicate circuitry (ie: the knock sensor amplifier and/or the TPS and MAP voltage regulators).

[omega_rugal]

so this thing may be $#!&ting on me too, i already changed the CPS sensor (PITA btw) and now starts almost instantly...when it feels like it

 

today it did again, i was driving to the store, stopped to get a few things and boom! i had to crank it for like 10 seconds to get it to start...

[cruiser54]

so this thing may be $#!&ting on me too, i already changed the CPS sensor (PITA btw) and now starts almost instantly...when it feels like it

 

today it did again, i was driving to the store, stopped to get a few things and boom! i had to crank it for like 10 seconds to get it to start...

Next time it does it, unplug the CPS and plug it back in.

 

You have cleaned the C101 I presume. 

 

Have you checked the CPS output yet? 

[omega_rugal]

can not unplug it anymore, since the last one did not have a conector anymore, i cut the wires and soldered them

 

 

there`s no C101 on my MJ

 

 

havent done that...

 

 

the thing is, the truck when it starts it works flawlessly, no random stall or revving up ord down or anything else related to a failing CPS, the problem is to get it to start, sometimes i turn it off and then try to start it again and boom nothing..

[cruiser54]

So, your CPS is hardwired, right? 

 

It would be nice to test it's output. Sometimes they are on the tipping point of generating enough AC voltage for the eCU to see. Hence, intermittent at low cranking speeds versus engine running speeds.

 

http://cruiser54.com/?p=50

[big66440]

So the "power latch relay" could be the cause of hard start/ long cranking condition for a cold motor? assuming everything else such as the IAT ,CPS, ECT sensors and idle control valve are functioning?.

[Ωhm]

I think you'll have bigger problems than hard start/long crank when B+ Latch circuit is faulty.

[big66440]

I think you'll have bigger problems than hard start/long crank when B+ Latch circuit is faulty.

I have to crank my engine for at least 5-10 seconds ( sometimes more than once) every time the engine is cold. I've done all of Cruisers tips, replaced the coolant temp sensor with a brand new Mopar one, tried several junk yard IAT sensors and not a single one has been able to fix my problem. Once my engine gets even the slightest bit warm it'll start right up :???:

[Ωhm]

Both IAC and TPS assume correct throttle plate setting. Did you try cruiser tip 14 – Restoring Throttle Butterfly Adjustment? Also never touch the fuel pedal when starting the vehicle. ECU will learn closed throttle over time. Does IAC reset when engine is shut off? You should be able to hear it. 3 to 5 seconds after key off.

[Jeep Driver]

 

I think you'll have bigger problems than hard start/long crank when B+ Latch circuit is faulty.

I have to crank my engine for at least 5-10 seconds ( sometimes more than once) every time the engine is cold. I've done all of Cruisers tips, replaced the coolant temp sensor with a brand new Mopar one, tried several junk yard IAT sensors and not a single one has been able to fix my problem. Once my engine gets even the slightest bit warm it'll start right up :???:

 

That is also the sign of a failing fuel pump or regulator.

[cruiser54]

 

 

I think you'll have bigger problems than hard start/long crank when B+ Latch circuit is faulty.

I have to crank my engine for at least 5-10 seconds ( sometimes more than once) every time the engine is cold. I've done all of Cruisers tips, replaced the coolant temp sensor with a brand new Mopar one, tried several junk yard IAT sensors and not a single one has been able to fix my problem. Once my engine gets even the slightest bit warm it'll start right up :???:

 

That is also the sign of a failing fuel pump or regulator.

 

Or leaking injectors when the engine is off. 

[big66440]

 

 

I think you'll have bigger problems than hard start/long crank when B+ Latch circuit is faulty.

I have to crank my engine for at least 5-10 seconds ( sometimes more than once) every time the engine is cold. I've done all of Cruisers tips, replaced the coolant temp sensor with a brand new Mopar one, tried several junk yard IAT sensors and not a single one has been able to fix my problem. Once my engine gets even the slightest bit warm it'll start right up :???:

 

That is also the sign of a failing fuel pump or regulator.

 

I've tried priming the heck out of the system many times to build pressure to diagnose the regulator and injectors without success, still does exactly the same thing.

[cruiser54]

 

 

 

I think you'll have bigger problems than hard start/long crank when B+ Latch circuit is faulty.

I have to crank my engine for at least 5-10 seconds ( sometimes more than once) every time the engine is cold. I've done all of Cruisers tips, replaced the coolant temp sensor with a brand new Mopar one, tried several junk yard IAT sensors and not a single one has been able to fix my problem. Once my engine gets even the slightest bit warm it'll start right up :???:

 

That is also the sign of a failing fuel pump or regulator.

 

I've tried priming the heck out of the system many times to build pressure to diagnose the regulator and injectors without success, still does exactly the same thing.

 

Are you checking the pressure with a gauge? 

[big66440]

^^^^^^Yes I sure have and they are within factory shop manual specs.

[cruiser54]

Okay, but what about hooking up the gauge when it is cold, read it. Turn the key to On and read it. Start the engine and read it. Shut the engine off and monitor it for an hour?

[big66440]

^^^^ Did all of that, everything checked out. I've tried 2 different fuel pressure gauges too and everything was within specs and held pressure even after shutting it off for at least an hour . The funny thing about this issue is it is a lot worse when I try to start it for the 1st time after it's been sitting all night ( or a very long period of time) and the weather is cold, it almost seems like the ECM isn't getting a signal to enrich the fuel injector pulse for a cold start. Once the engine gets even a bit warm I can start it up without a problem, it's less evident during hot summer months but still needs to be cranked longer than necessary when the trucks been sitting for a while ( overnight ).

[cruiser54]

^^^^ Did all of that, everything checked out. I've tried 2 different fuel pressure gauges too and everything was within specs and held pressure even after shutting it off for at least an hour . The funny thing about this issue is it is a lot worse when I try to start it for the 1st time after it's been sitting all night ( or a very long period of time) and the weather is cold, it almost seems like the ECM isn't getting a signal to enrich the fuel injector pulse for a cold start. Once the engine gets even a bit warm I can start it up without a problem, it's less evident during hot summer months but still needs to be cranked longer than necessary when the trucks been sitting for a while ( overnight ).

what is your CPS output voltage? 

[big66440]

Good question, I will check that as soon as I have a chance and report back. How long should I crank the motor to get a voltage reading? should the reading be steady?