Problems /w extended crank AFTER hot soak

OK, I need some help. Normally I can figure these out but this one has me perplexed. This is a customers rig.

1989 XJ 4.0/auto. Has recent OEM crank sensor after it died completely and was a no-start. It fires up cold perfectly. After driving a while, run into a store or something where it sits for 10-15 minutes and then you have to crank for 10 or 15 seconds or longer before it will start. After it finally starts, it blows a little black smoke and runs slightly rough. We have duplicated problem with scanner installed and fuel pressure gauge hooked up and aren't coming up /w much. Has RPM signal when cranking. Has good fuel pressure and HOLDS pressure once shut off. Coolant temp. sensor reads correctly. We do lots of jeeps and have parts lying around, we have substituted the following components with NO change: PCM (computer), another crankshaft sensor, ignition coil, module, distributor.

Any ideas?

John

Once it is good and warmed up, do you have the same hard to start issues or is it after it has ran and warmed up slightly?

Does yours have a regular carb with a mechanical choke? I had a '76 Dodge that would start just fine if it was cold or when it was fully warmed up. But would not start easily if it was just a little warm.

The solution I found was that I had to press the gas pedal all the way to the floor to start it because the choke was not opening up properly. By depressing the pedal down, it bypassed the choke.

I could be full of it, but it sound just like my old Dodge with. I think the choke is closed and you are not getting enough air, but plenty of gas, hence the black smoke.

Anthropy said:

Once it is good and warmed up, do you have the same hard to start issues or is it after it has ran and warmed up slightly?

Does yours have a regular carb with a mechanical choke? I had a '76 Dodge that would start just fine if it was cold or when it was fully warmed up. But would not start easily if it was just a little warm.

The solution I found was that I had to press the gas pedal all the way to the floor to start it because the choke was not opening up properly. By depressing the pedal down, it bypassed the choke.

I could be full of it, but it sound just like my old Dodge with. I think the choke is closed and you are not getting enough air, but plenty of gas, hence the black smoke.

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No offense, but WTF are you talking about? I am working on a 1989 Jeep Cherokee which has a multi-port fuel injected 4.0 liter engine. No carburetors here.

Avanteone said:

No offense, but WTF are you talking about? I am working on a 1989 Jeep Cherokee which has a multi-port fuel injected 4.0 liter engine. No carburetors here.

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I was thinking the same thing. :doh:

BTT

OK - you said you've just done the CPS, which was my first thought.

Does the "extended crank" just mean that it cranks a long time, or does it try to start and stumble a little? That would probably indicate a camshaft position/SYNC sensor going south...

I'd also check the grounds anyhow - especially the firewall strap. Heat will slighly degrade the efficiency of an electrical conductor, and I ran into a similar issue on my 87. I replaced the ground strap, cleaned up the patch where it is secured to the firewall, and stopped having problems.

I don't think I've run into any TPS issues that were heat-related, and I was able to start my 88 when that thing was going out anyhow (it had a flat spot at cruise.)

I'll chew on this some more and see what comes up - but that was my first thoughts upon reading this...

5-90

Thanks for the reply. I will double check those grounds as that is something that I did not think about. It does not try to start, you really have to crank on it a bit to get it to start. Cam sync is good on the scanner, and we have swapped in another just to verify.

J

Does it crank slow, then? The RENIX ECU won't "wake up" and start firing the ignition and FI until it sees 300rpm at the CPS - and heating up the starter will also make it crank slow (and therefore take the ECU longer to wake up...)

5-90

5-90 said:

Does it crank slow, then? The RENIX ECU won't "wake up" and start firing the ignition and FI until it sees 300rpm at the CPS - and heating up the starter will also make it crank slow (and therefore take the ECU longer to wake up...)

5-90

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No, it cranks good.

Any idea what changed BEFORE it got to you?

5-90 said:

Any idea what changed BEFORE it got to you?

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Customer purchased vehicle approx. 2 months ago. It's done this since he bought it.

J

vacuum leak easy to find with a propane torch

Awright - what data are you getting off the scanner, and what's your fuel pressure reading?

5-90