QuestionQUESTION: Hi there, I have a 96 jeep with the 4.0L engine that started playing up under acceleration. Backfiring, not revving past 3 grand when driving and surging all over the place under acceleration.
We put a fuel pressure gauge on the rail and drove it, found that fuel pressure was dropping under load, but not when in neutral - then it would happily rev out and hold constant revs. We dropped and cleaned out the tank (it did have some grunge) and cleaned the fuel pump sock, it seemed to run better a short while before playing up again. I also tried removing the gas cap to make sure it wasn't a vacuum problem.
Of note, flashing the computer seems to rid the problem for a short while, but then it soon returns. It also seems better when first started and driven cold. A mechanic put the analyser on it, but no faults were showing up. He seems to think its a fuel pump, but that to me doesn't explain the intermittent fault, and how it goes for a while after flashing the computer. I'm thinking Crank Angle Sensor? Does this have anything to do with fuel pump operation/ fuel supply? Appreciate any thoughts...
As jeep parts are so ridiculously expensive here in Australia i'm hoping you've come across this before and can point me in the right direction.
Thanks in advance and cheers from OZ,
Best wishes, Dave.
ANSWER: I am fairly sure that what you have there is an ignition problem. It is important, before trying to diagnose any suspected fuel problem, to do a complete ignition tune-up. I learned this working on English cars back in the '70s. If this does not solve the problem, I would try the crank sensor first. I have seen a lot of these fail. I would also be suspicious of the timing chain. A stretched chain can give these symptoms. And I have to admit, if I ever took down the fuel tank on any car I would replace the pump while I had it off. It's just too easy and cheap to wait and have to do the whole thing again later.
Good luck in OZ.
---------- FOLLOW-UP ----------
QUESTION: Thanks so much Mike,
My mechanic reckons he checked the ignition stuff, but if something was to be intermittent, what would be the likely culprit?
It's definitely a fuel pressure problem, the gauge shows a drop under load whilst driving. I'm leaning towards the crank sensor being intermittent.
Does the crank sensor regulate fuel flow / pressure whilst driving?
Cheers mate, Dave.
P.S. 170,000km on car and timing chain sounds ok and seems fine.
Answer The crank sensor is the most likely culprit. Fuel pressure always goes down under load, you're using more fuel then. "Checking" the ignition stuff is not the same as a tune-up. It really is important that there be no possible ignition problems before you start fooling with the fuel system. I have a 12 cyl Jag which I bought from a guy in Ohio. He spent untold thousands getting it to run right. He replaced all the injectors, the filters, both fuel pumps, the fuel tanks, and some other stuff. None of it helped. When he finally found someone who was familiar with the cars, they replaced the distributor and it was fixed. Never really needed anything done to the fuel system. The bonus is that I have a practically brand-new engine (only 48K miles) for cheap. Oh, and you can't hear the timing chain rattle in an OHV engine the way it does in an OHC like a Toyota. The only way to check it is to remove the distributor cap and rotate the engine forward and backward. If you see slack in the movement of the rotor, then the chain needs replacing.
