Start up question

[javelinkid]

Thought maybe someone here could help with a quick question. Every time I start my MJ, it cranks a little longer than I think it should before firing. It always starts, but just seems to crank long. Once it starts, it seems to "spike" in idle, then immediately idles normal. Just never has seemed quite right. Any suggestions? It's an '89 4.0, btw.

Thanks!

[Dirteatr717]

its because its a renix :yes: happens to me too :nuts:

[Akula69]

The reason for the long cranking time is the crankshaft position sensor, or CPS. It is a magnetic device that "watches" for gaps on the edge of the flywheel and reports back that information to the ECM. There is little you can do to make the cranking before start shorter, but if it seems extensively long (more then 8-10 seconds) you might want to replace it with the factor upgrade (where the wires from the CPS plug directly into the ECM rather then send the signal through the harness. I did the 'fix' on my son's 89, but unfortunately I cannot find the link for it.

 

The sudden spike in idle after the start could be a number of things, but I would make sure the throttle body is clean, and then ensure the IAC is absolutely clean and functioning. A good link for identifying and checking the variuos RENIX sensors is:

 

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

[Akula69]

Found the link for the CPS patch harness Tech Bulletin:

 

http://www.lunghd.com/Downloads_and_Links/CPS-87-90-update.pdf

[Squeegy410]

haha yeah my '89 4.0 is the exact same way. The PO got so worried he had replaced the starter with a brand new one and the plugs with platinums then the wires and a nice distributor :yes: haha so i had alot of really nice parts that were unnecessary but very welcome :D i wouldn't worry about the turn over time, but carry jumper cables if your battery starts to head south for the winter :D

[Oizarod115]

upon starting your jeep should see an instant rev to about 1500-2000 would be acceptable and then settling down to idle within 2-5 seconds.

 

long crank time happens, you can try priming the fuel pump by turning the truck to run for a few seconds then hitting start, instead of just rotating the switch directly from off to start. this will ensure you have full fuel pressure while cranking

[javelinkid]

Thanks for all the input guys! I'm gonna check out the thread you linked Akula69. Sounds like it might be a good read.

 

Congrats on all the new parts you scored with your MJ, Squeegy! :clapping: I always carry cables with me but winters here in Texas are pretty mild. Rarely gets cold enough where I'm at to even worry about CCA's

 

Oizarod, it does seem to be up around 1500-2000 right when it fires, then does settle down. I've tried priming by turning the key on and pausing before starting and doen't seem to make any difference.

 

Thanks again for all the input

[Eagle]

upon starting your jeep should see an instant rev to about 1500-2000 would be acceptable and then settling down to idle within 2-5 seconds.

When starting a Renix, the RPMs should NOT spike to 1500 or 2000 RPM, and certainly not for 2 to 5 seconds. That's a sign of impending problems. It shouldn't go any higher than about 1,000 RPM, and it should drop back to normal idle (+/-650 RPM) virtually instantly.

[Squeegy410]

The great and powerfull Eagle has spoken. I wish you were wrong sometimes haha cuz i want my jeep to not have problems :ack:

[Eagle]

Oh, I've been wrong more than I enjoy acknowledging. But I bought my '88 XJ new, and between that and a series of MJs I have logged nearly a half million miles behind Renix 4.0L engines. I have encountered the RPM spike upon startup multiple times, and it's my clue to dump a couple of cans of fuel system cleaner into the gas tank. The better fix is probably to remove the idle air controller and clean it, but it has to be adjusted when replacing so don't dive into that if you don't have the adjustment procedure documented.

[comanche09]

I thought that the prolonged crank time was due to the Renix computer waiting for adequate oil pressure before allowing spark?

 

Mine takes about 3-4 seconds of cranking to start hot or cold.

 

Once, started it goes to just under 1k rpm then settles to about 5-700 rpm in about 10 seconds.

[ParadiseMJ]

... you can try priming the fuel pump by turning the truck to run for a few seconds then hitting start, instead of just rotating the switch directly from off to start. this will ensure you have full fuel pressure while cranking

 

X2 here...I've been doing this for years. I had a Volvo 240GL Turbo...same deal. Turn to on...when the pump stops turn to start, no pedal.

[ComancheMan7893]

mine does the same thing i just wait a couple of seconds and then it goes away i don't worry about it ill get around to it

[javelinkid]

I think I may try to clean the idle air controller as Eagle suggests! I recently bought and sold a Cherokee with the same engine set up and it did not do this. I am sure it has to be something. It has done this as long as I have owned it (since it had 88K). I use sea foam on a regular basis, so most of the fuel system should be good.

Thanks again for all the feedback!

[Squeegy410]

I just poured some good lucas fuel system cleaner in a full tank of gas and burned all the gas up. my engine still likes to rev on start up. i really don't wanna take my iac apart and have no clue how. it appears as though its my only option. any help? (pictures and detailed instructions greatly appreciated :yes: )