McAdam

I just picked up a 1999 cherokee sport I got it for a song because it developed a no hot start issue. well, no warm start would be a more correct term. and it is slowly getting worse. I test drove it on friday and managed to drive around for 15 minutes with no issues, other than it not starting when we got back to the guys house. since then, it has gotten progressively worse. Now, it dies at stop signs with less than 10 minutes of idling.

symptoms...

when it is stone cold, it will start up with less than a full revolution. runs great. the longer you let it run, the worse it gets. I'm talking less than 10 minutes here, in Minnesota with the temp hovering around 25 and the hood up. Pretty sure it isn't vapor lock either. it has been producing more symptoms as time goes on. Now, when I rev it while sitting in the driveway, it will first act like it is going to stall out, then if I play with the gas pedal, it will rev, but the RPMs hang around 2000 to 2500 for about 5 seconds after I let off the gas. Then, when it finally dies, it will not re-start. It just cranks and cranks and cranks.

CKP and coil have been replaced, by me on saturday. I shortened the screws on the ECU cover, checked the MAP sensor and cleaned the contacts in the dist cap and rotor, checked PCV valve, and checked for vacuum leaks. nothing. When cranking and trying to start, it has spark (nice, blue spark that you can hear) and it has fuel. I pulled the plugs and they are wet. not soaking, but wet. It WILL NOT start with starting fluid! weird. It is NOT throwing any check engine lights/codes.

so, any ideas here?

ElMartillo

Coolant temperature sensor sending the wrong info to the ECU? If it always runs in cold mode, as it warms up the mixture will be too rich. My '89 has two temp sensors, one for the dash gauge, one for the ECU.

My non-expert opinion.

McAdam

I will check that and the IAT, easy enough. My only issue is the fact that starting fluid will not get this engine going when it stalls out. and I have spark, so that should not be the issue, unless the timing is REALLY out of whack.

McAdam

interesting. So, the IAT checks out okay. the coolant sensor, not so much. It is reading about 40K after the car is blowing hot air. and the longer I leave the multimeter leads on it, the higher the resistance goes! like, I can watch it climb as its running. With the coolant temp sensor plugged in, it will not re-start. with it unplugged, it will restart with some mild coaxing from the gas pedal. with it unplugged, it revs fine and doesn't hang at any RPM or stumble when I put some gas into it.

Obviously, I am going to replace it, but I am curious, does the ECU go into a default mode when it is unplugged? does it go solely off of the IAT? because if the resistance goes up really high, that means it's cold. and if I unplug it, it should just mean the same thing, so why does it run better with it unplugged?

also, where is the sensor for the dash temp gauge?

CCKen

I can tell you're a thinker.


The ECT Sensor is also the source for the temp gauge. It would be odd to have the temp gauge read normally (does it) and the ECT be defective enough to effect engine performance.


With the ECT not plugged in I think the PCM thinks its in Open Loop and all engine management reverts to preset programing set in the PCM. Could be why it runs okay with the ECT removed. With it connected (and sending corrupt data to the PCM) the PCM tries to run in Closed Loop but it is getting corrupt data to do so.


In Open Loop the Oxygen Sensors are ignored and in Closed Loop the O2S' are part of the PCM's engine management programing.


Generally, when you give it the gas, the TPS tells the PCM you are in something other than idle and the PCM will assume the Closed Loop mode.

freegdr

Codes were ?

McAdam

first, to answer freegdr, it did not throw any codes. nothing was outside the range of normal, it jsut wasn't operating normally.

It never got off the 100 or cold mark. Everything I have read says that these things have 2 separate sensors, so I thought the dash one just went out. I must have been reading posts on older models. If i had known there was only one coolant temp sensor on this year, I would have replaced that first and saved myself a couple of bucks!! but alas, the CPS would probably go out sooner or later and the coil had a few cracks in the exterior, so no harm really.


That sounds like what I was thinking. I am used to older vehicles (and megasquirt) that just default to -40 maps when you unplug the temp sensors, so I expected the same.


I think what was happening is that the spark plugs are just old enough that it was actually flooding and fouling out. Every time I checked spark, it was with an old loose plug I had laying around. I pulled a few plugs to make sure they weren't fouled and while they smelled of gas, they still sparked. perhaps they just weren't sparking with all the gas being poured in there because the car thought it was -30 or whatever.

With it sitting over night, it gave everythign a chance to cool down and get more inline with what the computer expected. plus, it gave the plugs a chance to dry off. then, when I ran it for a few minutes and put current through the sensor, it would tell the ECU that the engine was actually getting colder instead of warmer while the AIT was telling it the opposite. one sensor is telling it to enrich., the other is telling it to lean out. no wonder it caused some funky behavior.

thanks for the help, guys! I'll report back tomorrow (later today, I guess) and let you know if it fixed it.

CCKen

When the ECT is unplugged there is an open (high resistance) in the circuit causing a high voltage signal to the PCM, equal to/or greater than approximately -40* F.

McAdam

well, I installed the new ECT and it is MUCH improved. It still has a slight stumble upon acceleration sometimes, but hot restarts are no problem! I think the stumble may be bad/old gas. the guy I bought it from told me he hadn't driven in much at all in the past year.

I'll change the plugs and let'er buck.

CCKen

Sometimes stumble upon acceleration can be cause by a defective/failing ignition coil or the upstream O2S.


If you want to test your col instead of shotgunning it do this test and inspect it for cracks.




Test:





Cracks:





Also, verify the O2S Heaters fuse in the PDC is not blown.

ElMartillo

How much of this info is applicable to my '89 XJ base model with 6cyl?

McAdam

as stated in the original post, when I was throwing parts at it, I already replaced the crank position sensor and the coil. so, I'm pretty sure it isn't the coil.


the O2 sensor is probably a safe bet, seeing as how now that it is running right, it has since thrown a code. and that is the exact code it threw. O2 sensor heater. I will check in to that, thanks!

CCKen

None.


Edit: If you are having similar issues start a new thread giving as much detail as you can. One of the RENIX experts will help you out.

CCKen

Keep in mind that if the fuse is good the O2S itself, or the wiring for the heater may be defective.