89 4.0 Backfiring and bogging down

I went wheelin' at Uwharrie this weekend and a few times on the trails my engine would bog down like it was about to die and I'd have ZERO power. this would go on for a few minutes and then the jeep would run normall, then it would do it again at random. It seemed a bit better after I let it sit off for a while while the rest of the group went ahead and tackled the obstacles.

It also does this on the road at about 30-35 mph in 3rd (i think) gear. It won't accelerate at all and I get loud pops from the intake, like it's backfiring. I say intake because after the dusty trip at URE I could see dirt pop out from under my hood when it backfired. It also boggs down at random high speeds on the highway when I try to accelrate, however this is very infrequent.

My next problem is that it takes FOREVER to start. I let the fuel pump prime and build up pressure, then I start it, and it takes about 5-10 seconds before it cranks over, stumbling and sputtering until it finally starts up. A nice fellow with a white 91 XJ with a longarm kit told me that it was my check valve in my fuel pump.

Sooo Would replacing the fuel pump fix the problem? I have a new 02 sensor, plugs, wires, cap and rotor and everything else is in good shape. My injectors are also new.

If so, how much of an ordeal is replacing the fuel pump? Do I just drop the tank and pull it out?

check inside your distributor and make sure the pickup coil is not bent. I would replace your fuel filter as well. it might be your coil too.

Fuel filter is new as well. I think its the fuel pump just based on the long-crank times as well.

RyleyF said:

Fuel filter is new as well. I think its the fuel pump just based on the long-crank times as well.

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The crank times are just part of the Renix system. I have heard (on these forums) that the system resets the timing every time you start the motor. Yes the check valve in the pump could be gone, resulting in a longer start due to the lines backfeeding while the truck is off.

My 88 does the intake backfiring also, but only at cold temps before the engine has warmed up. I think mine is caused by the lack of a heat riser (I have a "cold air" cone filter intake). Mine stumbles like it wants to die, backfires, then runs fine.

as for your backfire, I'm not sure. yours could be caused simply by the fact that your engine almost dies, but saves itself. Figure out what is causing the bogging affect and you probably will fix the backfire

un plug your air intake temp sensor and see how it runs after that...seems i had a friend have similar problems and we started unplugging/swapping sensors to try and find the problem...we left the air temp sensor unplugged and it ran a lot better....let me know if that works cause I dont remember exactly what we did....

have you ever had the distributer re-indexed to prevent a crossfire condition? also sometimes the egr valve can hang open.

How do I re-index the distributor? I was also thinking its my fuel pressure regulator, and it'll still do the bogging down with the vaccuum hose removed and pinched off, which supposedly gives me more fuel pressure.

I know that the fuel lines are backfeeding to the tank, making me lose startup pressure, but would the fuel pump/regulator cause the backfiring and perhaps lean condition?

there are plenty of posts on how to re-index the distributor. just do a search. that was a common problem for 87-89 xj's.

You sure it could be the distributor? It runs fine sometimes, and then others it won't even idle right.

Check the fuel pressure regulator. Pull the vacuum line off and see if it is wet. If it is wet the regulator is bad.

How many miles on your XJ, people have gotten out of the habbit of posting 411 like that.

I replaced the distributor on my 88 XJ when it had 135,000 miles on it. I had more power, I went back and told the mechanic. He told me as the bushings wear in your distributor the timing is retarded.

On my 69 Camero with the 230 six cylinder had to have the distributor changed every 80,000 miles. I had to change it or else the points would not operate correctly. With our electronic ignitions we forget about these things since the computer will compensate for it.

I would say you have a timing issue. If you are unsure of the mileage on you distributor or it over 80,000 miles I'd recommend replacement. With replacement you will get the distributor indexed correctly.

Also you may want to replace the crankshaft position sensor (CPS) with the high altitude version, it will give you more advance which may help your starting problem.

Also checking time with a timing light may or may not give you an indication. If the rubber is bad in the cranshaft pulley the top dead center mark on will not be lined up correctly. The Harmonic balancer is something to consider, mine failed at 115,000 miles, it started eating belts and walking forward.

Worst case you have a timing chain the jumped one link and that why you are having problems. IF you change the harmonic balance it would be worth it to pull the timing cover to check out the timing set. That a rare thing on the 4.0 engine but it is possible.

It might be worth 1.0 hour of mechanic time for someone who understand the RENIX to troubleshoot before you throw parts at it. I had lost MPG and the mechanic found it to be a bad EGR valve, it was stuck open.

RyleyF said:

I went wheelin' at Uwharrie this weekend and a few times on the trails my engine would bog down like it was about to die and I'd have ZERO power. this would go on for a few minutes and then the jeep would run normall, then it would do it again at random.

My next problem is that it takes FOREVER to start. I let the fuel pump prime and build up pressure, then I start it, and it takes about 5-10 seconds before it cranks over, stumbling and sputtering until it finally starts up. A nice fellow with a white 91 XJ with a longarm kit told me that it was my check valve in my fuel pump.

Sooo Would replacing the fuel pump fix the problem?

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That nice fellow is probably right and if your fuel pump is 16-years-old, it's lasted well beyond the time it was designed for so replacing it will probably cure your engine's ills. The pump is mounted at the front of the tank so it's easy to access without dropping it.

RyleyF said:

I went wheelin' at Uwharrie this weekend and a few times on the trails my engine would bog down like it was about to die and I'd have ZERO power. this would go on for a few minutes and then the jeep would run normall, then it would do it again at random.

My next problem is that it takes FOREVER to start. I let the fuel pump prime and build up pressure, then I start it, and it takes about 5-10 seconds before it cranks over, stumbling and sputtering until it finally starts up. A nice fellow with a white 91 XJ with a longarm kit told me that it was my check valve in my fuel pump.

Sooo Would replacing the fuel pump fix the problem?

Click to expand...

Double post.

Yeah my XJ has 208,000 miles on it, so she's pretty old. I'm going to get my fuel pressure checked today as well. I was looking at a fuel pump at Autozone for around $90, is the check valve itself inside of the fuel pump? or is it in that big assembly that connects to the tank? I don't really want to get the whole shebang if I don't have to, I'd much rather save the $30 and clean out the old tubes and the pickup sock.

Also, if it was a timing issue then wouldn't it run bad all of the time? Its slowly getting worse.

A few weeks ago an Injector started leaking a LOT, so on my quick return home to fix it, going up a hill it would bog down and not let me accelerate. It eventually, after fixing the injector would bog down just every once in a while when accelerating a bit quickly. Since this past weekend its running like shit on and off at low speed after being on the trails for a few hours, and it wants to bog down in the highway and at about 25 mph almost every time I try to accelerate at more than a snail's pace.

It actually cranks up pretty well when it's cold, like in the morning, but it gets more and more cranky (literally) when it's hot. The harmonic balancer was bad before this summer so its all new, the old one was pretty shot and digging a bit into the timing chain cover.

The jeep has been hard to start i'd say within the last month or two. It happened right around the same time as I had to replace 3 injectors within about a month of eachother - they were the 3 originals left on the jeep. The other 3 were replaced a couple of years ago, and probably no more than 10,000 miles ago. I've bled the fuel rail several times to possibly get rid of the air, but no change.

didn't have a chance to get the fuel pressure fixed today. It was raining all day and so I couldn't get anything done.

If the original injectors were giving you trouble the others are most likely on the way out. I had one renix injector leak and it just seemed like the rest wanted to play along and started leaking. The XJ ran like crap until I replace them all. The original ones looked pretty gummed up.

The fuel pressure regulator is on the forward end of the fuel rail. When you check fuel pressure see how long it holds the pressure. If it bleeds down quickly you may have a leaking injector or regulator.


chris

It seems as if its even worse today. I hope that the newish injectors aren't on their way out. They're not even very old at all. I've checked them and while there's a bit of carbon around the plastic tip of the nozzle, the nozzles themselves are clean.

Whatever the problem is, it's getting progressively worse, and I don't think that the distributor would do that, or would it? I didn't have a problem with the original cap and rotor, and I didn't with the new one, until a few weeks later. However it was still starting hard when I changed the cap and rotor.

I checked my fuel pressure today... it's about 22 PSI. Which line is the return line on the fuel rail, is it the line near the firewall? I'm getting no pressure when I let the fuel pump prime either. So is it the fuel pump?

Edit: When pinching the return line the jeep wants to almost stall out and die. When vacuum is removed, there's no change either.

RyleyF said:

I checked my fuel pressure today... it's about 22 PSI. Which line is the return line on the fuel rail, is it the line near the firewall? I'm getting no pressure when I let the fuel pump prime either. So is it the fuel pump?

Edit: When pinching the return line the jeep wants to almost stall out and die. When vacuum is removed, there's no change either.

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IIRC your psi should be about 31-32 with vacuum attached and when vacuum is removed the pressure should go up about 8-10 psi.

Seen as your Jeep wants to die i would say that its is most likely your pump or the filter.

Check the filter, and if it's dirty replace it and retest the fuel pressure.

If there is no change, then I would replace the pump.

I put in a new pump yesterday. All problems are gone and it starts right up. Fuel pressure is a steady 31-32 and its about 40 with the vaccuum removed.