Engine miss - riddle me this...

[Mudderoy]

A year, or more, ago I was in the midst of trying to figure out a running hot on the highway problem. One of the many suggestions was that due to the increased capacity and flow rate of the high flow components that the coolant was passing to quickly through the system and the radiator wasn't having time to cool it.

Digging around I found there was an easy solution that could be put in place and taken out. Great for testing. It was a restrictor. Basically a nylon cutting board and a 2" (I don't recall the size now) disk cut out of it. Then a 3/8 to 1/2" hole in the middle. You just take off the upper hose at the thermostat housing and place the restrictor in the hose and let the housing push it back down the upper hose. The idea being that the coolant coming out of the radiator would have to slow down to fit through the 3/8" to 1/2" hole. This was much quicker than changing out all the high flow stuff.

http://www.go.jeep-xj.info/Restrictor6.jpg :link: (http://www.go.jeep-xj.info/HowtoRadiatorRestrictor.htm)

I made the restrictor and placed it in the hose. Fit great! I started up the 1998 and I had one hell of a miss. If memory serves correctly I believe it even through a code! I do not recall the code, it was so long ago. I removed the restrictor and reset CEL.

Fast forward to me picking up my beloved 1998 Jeep at the radiator shop. It had a miss! It's had a miss (off and on) since. I've tried various things but it has not resolved the problem. When I get out of idle it seems to run fine, at idle is sounds bad and at times has even shook.

I've noticed since picking up the Jeep that the coolant temp hasn't been very high, no where near it used to be. I figured this was do to the cooling system working MORE correctly since the transmission shop replaced a rear freeze plug. Still it seemed a big change.

Finally (Sunday night) I replaced the thermostat with a 195 degree Stant. I broke my high flow thermostat housing in the process, so I had to go back to the original. I would have changed the thermostat sooner, but I was having issues trying to get another RobertShaw.

The results have been remarkable. There is no miss. It was just like what happened with the restrictor.

I was getting a P0301 after driving about 50 miles. Occasionally I would see a P1281, which says to CHANGE THE THERMOSTAT when you look it up. :rotfl2:

I'm only into the 2nd full day of driving it but so far none of the knocking or jerking is there. I had some slight knocking yesterday, but it was about 10% what it had been. Today, nothing.

Does anyone have any idea why a restrictor or a thermostat stuck open (I'm assuming open since the engine was cooling MUCH better than normal) would cause a miss-fire?

[4.3LXJ]

That one is beyond me. Maybe the gremlin died. :D

[Mudderoy]

That one is beyond me. Maybe the gremlin died. :D

You don't have any idea about why it missed with the restrictor either?

[4.3LXJ]

You don't have any idea about why it missed with the restrictor either?

This is kind of a situation where I need to be there because it is not the thing you would expect. But if I had to guess, the restrictor was causing low flow and making hot spots, hence the pinging in only one or two cylinders. The block is long, so the rear half gets much hotter than the front, a common situation with a straight six. According to my wiring diagram, you don't have a knock sensor, which surprises me somewhat, so you could get a knock from a hot spot preigniting the mixture. It seems to me that you would need adequate flow to keep the whole thing cool and that in a low flow situation, the path of leas resistance is through the front of the engine. But it is kind of a guess to me.

[Mudderoy]

This is kind of a situation where I need to be there because it is not the thing you would expect. But if I had to guess, the restrictor was causing low flow and making hot spots, hence the pinging in only one or two cylinders. The block is long, so the rear half gets much hotter than the front, a common situation with a straight six. According to my wiring diagram, you don't have a knock sensor, which surprises me somewhat, so you could get a knock from a hot spot preigniting the mixture. It seems to me that you would need adequate flow to keep the whole thing cool and that in a low flow situation, the path of leas resistance is through the front of the engine. But it is kind of a guess to me.

I like this but the thing that was really strange was it wasn't a situation where I took a hot engine and put in the restrictor. It was cold. I didn't run it for 5 minutes before getting the miss, it was immediate! It made NO sense to me at all, and still doesn't. I could be remembering it wrong of course.

Now the hot spot you're talking about and pre-ignition makes sense however the mis-fire was on cylinder 1 and that is the closet to the thermostat!!!! My head hurts. :rotfl2:

[4.3LXJ]

I like this but the thing that was really strange was it wasn't a situation where I took a hot engine and put in the restrictor. It was cold. I didn't run it for 5 minutes before getting the miss, it was immediate! It made NO sense to me at all, and still doesn't. I could be remembering it wrong of course.

Now the hot spot you're talking about and pre-ignition makes sense however the mis-fire was on cylinder 1 and that is the closet to the thermostat!!!! My head hurts. :rotfl2:

Look for a dead Gremlin. :p

That is the problem, I really don't have a good explanation either. Anything I would come up with for this would be a WAG (wild :ass: guess)

[Mudderoy]

Look for a dead Gremlin. :p

That is the problem, I really don't have a good explanation either. Anything I would come up with for this would be a WAG (wild :ass: guess)

Well I feel good in a way, at least I'm not the only one that doesn't understand it.

[4.3LXJ]

If you had coil on plug, then you could attack it from an ignition stand point. But you don't, so that rules it out. The only way you could possibly figure it out is to have it happen again. Can' fix stuff that ain't broke

[Mudderoy]

If you had coil on plug, then you could attack it from an ignition stand point. But you don't, so that rules it out. The only way you could possibly figure it out is to have it happen again. Can' fix stuff that ain't broke

I have to put the firewire wires back on, put the new CPS back on, put the newer cam shaft position sensor back in the cap...

:(

[4.3LXJ]

Good luck. Jeep, forever under the wrench.

[olliewagon]

A bad or sticking exhaust valve will create same simptom. And the thermostat housing is the out put to the rad , blocking that hose should make it run hotter i believe . Plus i needed to post something lol.

[Mudderoy]

Well the world made a little be more sense today when I left for work the engine was missing like it was missing before. Thermostat is working as good as it was Sunday, so I had a few days of relief that the problem was resolved before it returned.

[4.3LXJ]

Gremlins I tell ya :D

[OhJeepers]

It's possible that the different temperature of the motor while its running could have maybe cause some thing mechanical to warp or some thing? I've heard the valve stems in jeeps can warp really easy( unsure if there's truth to that) if so thou on could be sticking long enough to miss fire but also work free long enough it doesn't do it all the time maybe.