'87 Xj 4.0l - Oscillating Rpm / Usually When Warm

[talentjeep]

I recently took my XJ into a shop. They cleaned the throttle body, checked the temperature sensor (not the idiot gauge one), replaced the rotor & distributor cap, new motor mounts, engine fan, radiator, plugs, wires, oil, air filter and did a tune-up. They even checked it's temp at idle and said it was 212 deg F or so.

My Jeep ran fine until it got fully warmed up. At 65 mph the engine would cut out for half a second intermittently. I drove about 20 miles at freeway speeds. When I got to the exit the rpms would go from 1500 to 3000 despite my pedal workings. Gas kept it going but revved it dangerously high and it still jerked violently. My battery totally died and I coasted to a gas station. Tested battery and alternator, battery was dead. Bought new battery...

It still jerked a bit in the low RPMs when I drove it the mile back to work. When I got off work it was cold and ran pretty well. It started "hiccuping" again on the on ramp and then behaved fairly well until the exit 7 miles later. Same problem. Jerked and kicked and revved. Forced it all the way home and now it dies at idle.

This was the problem I took it in to have fixed and I'm going to take it back to the shop. But does anyone have any ideas? Has anyone heard of this symptom before? I've gone over the thing myself several times.

????

[talentjeep]

Bump.

Anyone?

[amgqmp1]

Sounds like the ECM. I'm not sure how you check the ECMs on our old Renix setups...but I do recall seeing it under the dash on the drivers side.

Does unplugging the connector for the TPS and/or the IAC from the side of the throttle body get it idling again?

[talentjeep]

I disconnected the TPS & IAC. No effect, engine still wouldn't start.

I tried connecting each one individually with no luck.

I reconnected both of them and pulled the hose from the exhaust manifold off of the air box so the engine could only pull cold air. It started immediately!!

Is this because a flooded engine can't start on it's own air? Any ideas?

[talentjeep]

It works! Just pulling off this hose made the difference. My XJ now starts right away and runs fine. Put 40 miles of highway/street driving on it to be sure.

Luckily since it's an '87 I don't have to do emissions in my county (21+ years are exempt). Does this mean I had a bad EGR valve or that the air box was pulling in exhaust gas at the wrong time?

[Fir3Chi3f]

hmmm, I might need to try this. 87 was a good year by the way I'll post back when I can with my results

[talentjeep]

It's too early to tell but it may have decreased my gas mileage a little. I have read several places that removing EGR will cause 10% fuel efficiency reduction.

The needle seems to be going down a little faster than normal but I had it idling for 45+ minutes when I first got it going again. I'll post MPG as I figure it out. Previously I got about 18-20 hwy / 16-17 city.

When I replaced air filter I saw a lot of black, sticky gas smelling stuff on it at the front of the air cleaner. I thought it probably hadn't been changed in a long time and that it had sucked in a lot of burnt coolant when I initially killed the radiator. Knowing that hose made such a difference makes me think this has been a problem for this Jeep for awhile but the previous owner never noticed it.

Here's the old air filter:

[Fir3Chi3f]

I relation to the egr valve, here is the link to my other thread so I don't have to repost here. Even though an egr valve could be good from this test it may still need to be replaced.

I might end up replacing it regardless of know if it is bad or not because it is probably the original and the previous owner of my jeep probably didn't know.

Removing that hose you mentioned maybe a good test for me because I have been getting very poor mpg and if it doesn't change with removing that hose that would be a good indicator.

[talentjeep]

I am interested to see if it fixes your problem.

For me that hose blows out a fair bit of hot exhaust air, combine that with the air being pulled by the engine at high rpm and I could see it pulling too much exhaust gas through that dinky little valve on the front of the air box.

I've been theorizing that if a catalytic converter were clogged/blocking airflow that maybe the extra flow backs up through the manifold and into the air cleaner

I do have a slight hissing sound coming from my exhaust pipe. Since I don't have emissions anymore I'm going to remove my cat next weekend I think and put a high flow muffler on it. Then I'll try reconnecting the hose and see what happens.

[Fir3Chi3f]

regardless of the state of the cat. I don't think the egr valve should allow any exhaust gas in when it shouldn't. From my understanding it should only allow exhaust in in upper rpms at a constant speed.

I'm at school and don't drive much. So it might be a while before I get out to the parking lot.

[Fir3Chi3f]

So I got out today and removed that hose that you were talking about talentjeep and it did run alot better Shifting was easier, if I messed up a little it wouldn't jurk my passengers around. I bet it is the egr valve in my case at least.

Any word about the cat. talentjeep? find the problem?

[talentjeep]

No word on the cat yet. I need to hit it with a hammer. But I get paid this friday so off to the muffler shop!

Gonna spend too much money. I made another thread about re-routing exhaust to the side but no one seems to care/think it's an interesting idea.

I guess I'll stay stock and keep the axle hump but get a better cat and muffler.

Best suggestion was do 90 deg bend and route it up through hood over roof. In case I want to do 5' deep water...

[Fir3Chi3f]

Link me to that thread so we can talk about the re-route there, but I'm going to replace my egr when I get around to it and I'll post back my results.

[talentjeep]

Originally Posted by Fir3Chi3f

Link me to that thread so we can talk about the re-route there, but I'm going to replace my egr when I get around to it and I'll post back my results.

Here's my strange idea exhaust thread:

Relocating Exhaust

As far as the engine goes, it's running really well. I almost believe the blonde girl I bought it from when she said "The engine's been rebuilt but I don't know when that was...." Gobs of power. Sounds great, idles smooth.

Bad News is that I think I might be getting 15 mpg. Almost down to empty on this tank so I won't know until I go through another full tank and save receipts.

I'm almost willing to make the trade though. When it revvs it's loud enough to hide most of the other weird noises.

[Fir3Chi3f]

Originally Posted by talentjeep

Here's my strange idea exhaust thread:

Relocating Exhaust

As far as the engine goes, it's running really well. I almost believe the blonde girl I bought it from when she said "The engine's been rebuilt but I don't know when that was...." Gobs of power. Sounds great, idles smooth.

Bad News is that I think I might be getting 15 mpg. Almost down to empty on this tank so I won't know until I go through another full tank and save receipts.

I'm almost willing to make the trade though. When it revvs it's loud enough to hide most of the other weird noises.

Dido on the mileage, I think I'm getting around 16 just from calculating with the trip. and what the pump says in gallons. Not the most accurate method on my part but it should still be more than a consistent 16mpg.