cherokee runs rough, backfire, missfire, etc

I have a 1989 jeep cheroke that Backfires, misfires, runs really rough for the first few minutes after startup, sometimes smooths out a bit but not completely and is drivable. cylinder 2 seems to be dead, unplugged the injectors one by one to see if it changes the way it runs, it bogged down on all but the number 2 cylinder. It goes through a quarter tank of gas in about 30km, I thought it might be the fuel pump dying as it is very loud and erratic sounding, I have a new one to replace it on order. I just wanted someone with more experience to help me out.

If it helps at all, I can post a video of how is sounds when its running to give you a better idea.

So far, I have replaced the
distributor cap and rotor
ignition control module
fuel pressure regulator
vacuum lines (re-routed/repaired)
exhaust manifold
O2 sensor
crankshaft position sensor
ran a test on every sensor I could find (think I got em all) - all tested GOOD
fuel filter

Search for Cruisers renix diagnosis and refresh threads. That is the remix bible. You will learn a lot about your XJ and grow to love it even more.

Forgot to mention, I'm not sure if this is in the right sub-forum, if it isn't, could a mod move it?

Note from DJ
Here is the link to cruiser54 Renix Bible of tips and tricks.
Thanks cruiser
Revised on 03-13-2014
http://www.cherokeeforum.com/f51/cruiser54s-mostly-renix-tips-153657/

I hope you ordered a Bosch fuel pump not an Airtex: Airtex has a bad repletion of a high shot term life span.

Remove your #2 spark plug and tell us what it looks like.

Where in BC my wife has family on the Sunshine Coast as well as the Vancouver Island?

While you have the spark plug out, do a compression test on at least that cylinder, if not all.

Double check that you don't have a miss wire on your spark plugs. IIRC the firing order is cast into the intake manifold.

I have read through those renix tips, I cleaned all plug wires and grounds, etc. they didn't have any bosch fuel pumps at the place I ordered it from, so they gave me an acdelco one instead. The spark plugs are definitely wired correctly, same with the injectors, it also seems to be getting good spark. I will do a compression test today after I change the fuel pump, I will let you know the readings.

I've had this issue... first thing you should check is fuel pressure at the rail. There are two issues I've seen that cause this in order of likelyhood:

1) the little rubber hose that connects the fuel pump to the bulkhead connector rots after 20+ years, and 10+ years in ethanol gas. The rotting will cause it mostly pop off, so it gets some pressure, but usually never gets to be more than about 15PSI or so, the renix needs at least 30PSI to reliably inject fuel

2) Your fuel pressure regulator (on the front of the fuel rail) may be bad, usually this just causes it to run rich, the way to check it is pull the vacuum line off the front and if it's bad gasoline will be filling the tube. I think this is the less likely, but the easier to fix of the two problems.

Here's my thread on the same issue:
http://www.naxja.org/forum/showthread.php?t=1108357&page=2

I checked the fuel pressure, it was getting around 23-25psi at idle, and when I unplug the pressure regulator, it was getting closer to 30. I would also like to mention, around 8 months ago, it was having a weird problem where it would rev up to around 4-5k rpm when I started it and for some reason, changing the pressure gauge fixed that, I just swapped it out with one from another cherokee, so I don't necessarily think that that's the problem.

I checked and cleaned the sensor grounds. I'm still thinking it has something to do with the fuel pump, but the one they sent me (ordered from a local store) did not fit in my jeep, it was way too small, and they could not find the pump that I needed on their computer, anyway every time I start it, it will run like crap for about 5 minutes, at which point it seems to either smooth out, or the computer compensates for some problem, it seems that when I first start it, its dumping so much fuel it is barely able to idle, then the computer compensates for something and it smooths out a bit (still goes through gas very quickly) My question is, what are some things that could cause it to do this? why is it able to sorta smooth out? could it be a rings? valve seals? timing chain? If I have an idea of where to start, then I might be able to diagnose it better. I don't really have that much experience with this sorta thing, that's actually why I posted here, so any help is appreciated.

I checked the fuel pressure, it was getting around 23-25psi at idle, and when I unplug the pressure regulator, it was getting closer to 30. I would also like to mention, around 8 months ago, it was having a weird problem where it would rev up to around 4-5k rpm when I started it and for some reason, changing the pressure gauge fixed that, I just swapped it out with one from another cherokee, so I don't necessarily think that that's the problem.

I checked and cleaned the sensor grounds. I'm still thinking it has something to do with the fuel pump, but the one they sent me (ordered from a local store) did not fit in my jeep, it was way too small, and they could not find the pump that I needed on their computer, anyway every time I start it, it will run like crap for about 5 minutes, at which point it seems to either smooth out, or the computer compensates for some problem, it seems that when I first start it, its dumping so much fuel it is barely able to idle, then the computer compensates for something and it smooths out a bit (still goes through gas very quickly) My question is, what are some things that could cause it to do this? why is it able to sorta smooth out? could it be a rings? valve seals? timing chain? If I have an idea of where to start, then I might be able to diagnose it better. I don't really have that much experience with this sorta thing, that's actually why I posted here, so any help is appreciated.

I checked the fuel pressure, it was getting around 23-25psi at idle, and when I unplug the pressure regulator, it was getting closer to 30. I would also like to mention, around 8 months ago, it was having a weird problem where it would rev up to around 4-5k rpm when I started it and for some reason, changing the pressure gauge fixed that, I just swapped it out with one from another cherokee, so I don't necessarily think that that's the problem.

I checked and cleaned the sensor grounds. I'm still thinking it has something to do with the fuel pump, but the one they sent me (ordered from a local store) did not fit in my jeep, it was way too small, and they could not find the pump that I needed on their computer, anyway every time I start it, it will run like crap for about 5 minutes, at which point it seems to either smooth out, or the computer compensates for some problem, it seems that when I first start it, its dumping so much fuel it is barely able to idle, then the computer compensates for something and it smooths out a bit (still goes through gas very quickly) My question is, what are some things that could cause it to do this? why is it able to sorta smooth out? could it be a rings? valve seals? timing chain? If I have an idea of where to start, then I might be able to diagnose it better. I don't really have that much experience with this sorta thing, that's actually why I posted here, so any help is appreciated.

In your first post, you already isolated #2 cyl. Why haven't you pursued that? Check compression. Swap the injector with a different cylinder and see if the problem moves. Seems like you are chasing things that would affect all the cylinders.

I don't actually think that the cylinder is dead, I just assumed that because when I unplugged the injector for that cylinder, the engine didn't seem to bog down, but when I did it again a few times recently, it would take a second, but it did drop in RPMs, I think the problem is that its just dumping so much fuel into the engine, I don't know what could cause this (except from the obvious, map, O2 sensor, etc). When I had the fuel pump out yesterday, the lines all seemed good, so I don't think its that, so it could either be a failing sensor or wire (from what ive seen though, they look okay), etc. I would also like to mention that I examined and repaired/replaced all the vacuum lines, so that shouldn't be an issue. One other thing, the brake booster seems to be shot, whenever the jeep is running, I can hear a loud hissing sound coming from the brake pedal area, it goes away when I compress the brake pedal, if it is the booster, I can get a new one from a non renix cherokee, I was just wondering if it would be compatible since its not for a renix. I will run a compression test tomorrow if I can get a gauge and let you know what I find.

My first two guesses would be the CPS, the second the MAP and/or vacuum.

The CPS can be temperature sensitive but in my experience they tend to maybe work cold and get worse as they heat up, but anything is possible and yours may act up cold and get better as it heats up. I always check to see if the CPS wires are cooking on the exhaust manifold, seen this a bunch of times.

The MAP has a lot to do with how rich it runs. The vacuum tube for the MAP often rubs through on the bottom. People sometimes screw up and plug the MAP vacuum line into the wrong opening on the side of the TB, only one hole goes all the way through for vacuum.

Easy to bend a pin plugging the MAP connector in.

I'd plug off the vacuum booster opening and see if anything changes. And check the torque on the intake/exhaust manifold bolts. I'm thinking a little vacuum leak here, a little there and it all adds up and may screw with the MAP function/fuel metering. My 87 had a small hiss in the brake booster (for around three years) that never caused any issues I ever noticed.

Some other possibilities that come to mind is a leaky injector or a bad diaphragm in the fuel pressure regulator (you said you swapped in a used part?) dumping fuel into the intake through the vacuum line.

One oddity I've only seem one time, was a charcoal canister completely full of fuel and when the ECU would open the purge solenoid it would dump a chit load of fuel into the intake.

You may want to also do a search and read through the Renix files. They aren't as organized as Cruisers check list, but seem to cover almost any type of problem you are likely to come across.

Last tip, don't assume it is the sensor, better than half the time it is a connector, a splice or even a wire that is causing your issues. The Renix engineers did some things well, the lowest bid wiring wasn't one of them. Troubleshooting by buying or swapping in sensors may be a waste of time and money. And you may even be shooting yourself in the foot because many of the after market (or even the newer OEM) sensors you are swapping in my not be as good as the original factory parts (from the late 80's). The vast majority of the parts in my 88 are still original, some are out of my 87 that was rolled and others are junk yard OEM factory parts.

Decided to check the spark plugs today, glad I did, because the problem does indeed lie with the number two cylinder. the plugs in all cylinders look great, except for the number two cylinder, the plug in that one was caked with burnt oil and carbon, I cleaned it, but it still runs rough, so my guess is that it had something to die with either the rings or valve seals.

Ran a compression test, got:150 160 160 150 175 150
Cylinder 2 is much higher then the others, I think its caused by the valve seals
Leaking in that cylinder as there was a lot of oil and carbon on the spark plug. A few years ago, the PO said that it had a problem with overheating and got really really hot, enough to make it die, it did this more then once and I think I'm paying for it now, so I will replace the valve seals and see if then changes anything.

Note from DJ
Before you go to all the work and time replacing your valve stem seals if I were you I would very strongly consider fixing the problem mentioned in your post #12.

QUOTE: “One other thing, the brake booster seems to be shot, whenever the jeep is running, I can hear a loud hissing sound coming from the brake pedal area, it goes away when I compress the brake pedal”

That vacuum leak is a very large problem that your engine management system is trying to correct for. You need fuel and air in the proper ratio plus a good spark for a proper running engine. That vacuum leak is introducing a large amount of uncontrolled air into the system that the ECU is trying to compensate for by increasing the fuel ratio. This will account for your poor gas mileage, fuel smell at tail pipe and rough running engine in my opinion for what it’s worth.

If I were you I would try a simple quick fix by remove the hose between the intake manifold and the brake booster at the brake booster end; do this with some caution, that check valve that is inserted into the booster at the end of the hose is plastic and will break. Now remove the check valve from the end of the hose by using a turning and puling motion. After removal of the check valve look for something round like a wooden pencil perhaps to plug the opening in the hose this will eliminate any vacuum leak related to the booster.

Now start your Jeep but DO NOT DRIVE IT! You now have no power brakes let it idle a little bit then give it some quick boost of acceleration a few times. Let us know if your idle quality improves after a few restarts.

Thing is though, the problem started BEFORE the brake booster started leaking, and I tried what you said, and it didn't run any better with the brake booster disabled. I can get a newer booster, but its not from a renix, would it still fit right?

Strange to have just one bad valve stem seal. Have you switched the injector, spark plug, and wire to a different cylinder, to make sure they are all good?

I just went through this issue.
Turned out i had a cracked cyl head and a blown headgasket between numb 2 and 3 cyl..

Wouldn't that decrease the compression though? I'm getting higher compression in the problem cylinder, but anything is possible.