Hey all, I've got a '98 that's giving me some issues starting. It's got 207k on the jeep and ~2k on a remanufactured ATK 4.0L from Napa. It starts fine cold and runs normally but when I park it and let it sit for even just 5-10 minutes, it won't fire back up, it just cranks and cranks with no indications of combustion. If I shut it off after the first start and drive and almost immediately try to restart it back up, it will fire but it starts very rough and just barely keeps itself alive. I've tried a few basic things to troubleshoot but I'm hoping someone has some input on where to look next, I was betting the farm on a leaky injector and that didn't fix it.

-I can smell gas very strongly around the car when I crank, so my thought is that it's getting (or at least leaking) fuel into the cylinders.
-As far as spark, the interesting thing to me was that after a few long cranks I figured I would make sure I didn't flood it and try to clear it out so (before I knew about the 100% pedal clear flood) I just unplugged all injectors and tried to crank it normally and it fired! Obviously only woke up for a second to burn what fuel was in the cylinder but seems like a good sign that it's getting spark like it should. Plug the injectors back in, immediately back to just cranking.
-I have an 01 with the failed fuel pump check valve that I have to prime 3 times and it starts like a champ EVERY time, so i'm fairly confident that it's not the fuel pump/pressure, but I hooked a pressure gauge up to verify pressure. 49psi on the dot while the pump primes, drops to 40ish after the pump stops and holds for a few seconds, slowly decayed down to ~30 over the course of the minute that I left it hooked up. The 01, after i build pressure, will decay much quicker down to 0 essentially, as I would expect, so different behavior there.

Based on all that, I figured a leaky injector(s) might be the culprit, and this still had the stock 1 holes, so I upgraded to a set of 4hole EV6's with adapters.
-Same issue, starts cold fine and runs well. Pull it back in to the garage and let it sit for 10 mins, crank crank crank...
-100% pedal clear flood mode did fire it up at this point, I'm sure it would have before but I didn't know about it until recently.

That's as far as I was able to get so far, based on reading around, I was going to try a couple sensors next to see if that helped
- maybe crank position sensor is getting heat soaked? seems odd that it would start with the injectors disabled if this were the case though?
-idle air control or TPS - this seems most likely to me, my initial thought was no start due to excess fuel but maybe it's not enough air, still leading to rich in cylinder. I don't have a tach so it's tough to watch RPM but I'll try to watch it on the OBD reader maybe the next time i fire it up and see how much that moves around.

Does anyone have any thoughts?? I'm willing to fight it a little longer before I break down and take it in but if these next couple things don't work out, i'm stumped!

Thanks,
Kyle

 

Bad coil? Verify you actually have spark when cranking. I'm suspicious of the crank sensor too, but be careful throwing parts at it. The quality of chinese parts is pretty low and you might be swapping out a good part with a crap one.

 

So to follow up on the crank sensor, it looks like one of the M7 bolts that holds it in is missing...I don't know much about how sensitive they are but seems like this might cause enough of a proximity issue with the sensor that could get worse with temp so I'll try to track one of those down before replacing the sensor. I also DMM'd the sensor pins ~6 MOhm across B and C and OL across A and B. This doesn't match what I've read for other folks saying it should be but that's cold. I'll retry the measurement once it's warmed up. Also added a few pics of the throttle and IAC, I took those off to clean them. doesn't seem like as much carbon on the IAC as it is just other crud.

 

Yes, the sensor needs to be correctly aligned to work right. That resistance spec you found for the crank sensor is for the older 2-wire Renix sensor, not the newer HO engine sensor.

 

Update:
Throttle & IAC cleaned - no luck
IAC replaced - no luck.
I found the crank sensor bolt, but struggling to get that in, i think the threads might be messed up or the hole blocked. With all the extensions and universal joint I can get on it but can't get it to bite.

I backprobed the TPS while it was engine off and while idling, getting 5.0 V power and ~0.78V at idle, ~3.88V at WOT. This is within 0.1 V on either end of what the '01 is showing and seems to be smooth (tough to tell, I only have a digital voltmeter). I haven't checked spark by pulling a plug yet but everything I'm seeing is pointing to an AFR issue since even hot, it will start with some help. Still starts first time cold (after overnight or really long sit), won't restart after that until it sets for a long time. The only way to get it started after the initial start is 100% pedal (injectors off) and that works every time.

A New-ish symptom that is now present is that I get a pretty noticeable RPM dip when i tip in from idle. It hasn't stalled yet but I can tell it's bogging down.

FWIW the injectors that I got from Precision Auto Injectors are the 0 288 155 903's, they have great reviews, even a few folks recently dropping them in 98's with no issues but I realized i didn't clarify that in the initial post.

 

FIXED. I started it up cold a few nights ago and kept a closer eye on all the data on the scanner. The ECT sensor started out slightly lower than ambient and only really warmed up to 50 C or so, and fuel trims were in the -20ish% range but not off enough to throw a code. LOTS of drop outs to -20 on coolant temp, and trying to start when reading -20 was the issue, I wiggled the wire and got it to read 50ish, started up fine.

I just replaced the sensor (MOPAR part from Moparpartsgiant.com) and the connector (NAPA) and it runs and starts like a champ.

Wish I would have paid attention to that ECT reading a little sooner...at least I got a new IAC valve for my trouble...

 

I've seen where the wires to the ECT are stretched a bit tight and sometimes it's the wire that's breaking inside.

 

yeah, could have been honestly, the wire was pretty short and tight. There is a good bit of crud built up on the sensor pins to the point where the resistance reading was all over the place with a DMM solid on the pins, so i'm thinking it was a combo of both sensor pins and connector crudded up. I'm adding a good once over of the harness and all sensors to my list after this adventure.