Runner 03, You’re Welcome! I Hope It Works For You.
Great story. The speedometer information got me thinking. Are you a writer, by profession?
Please let us know what happens. We can fine tune our advice from the feedback we receive.
I wonder what kind of an engine this model year used…Who made the injection system, the ignition…So many possibilities…
On a '94 Jeep model, I hope that we can rule out the original Whippet engine from the 1930s!
I believe that Jeep stopped using that engine at least a few years prior to that model year, but…Quien sabe.
As everyone else has said… There are a number of possible causes. One of the likely causes that hasn’t been mentioned yet is a valve problem (sticking valves, specifically). This will often give the exact symptoms you are seeing (intermittent power loss under load). The number of miles on your engine (assuming it is the original engine) makes this possiblity more likely.
Of course, the first step in diagnosing this as a valve problem is ruling out the other (less expensive) problems already mentioned (fuel delivery, etc.). If you are lucky, the mechanic looking into it will find a less expensive cause.
My guess is a failing fuel pump, especially if there are long intervals between filter changes; or if the filter has never been changed.
A pump can be erratic in nature. It may work fine for a week, month, or 2 months and then act up for a minute, an hour, or permanently.
If the Jeep were mine I would test the electrical current being drawn by the pump. If the current draw appears to be a bit excessive or it varies a bit the pump could be the problem.
For a possible solution, one needs the symptoms. The symptoms for this 1994 Jeep Wrangler, with a 4 cylinder rebuilt engine, are:
Intermittent periods of loss of acceleration, and loss of power during cruise at highway speeds, with backfiring in the exhaust. Sometimes the speedometer stops working…sometimes (not, always?) during problem time.
After the fuel injection system was cleaned, it was trouble-free for 70 miles (coincidence?).
We have been concentrating on a fuel problem. Might it be a spark problem?
Start with a question. What causes backfire in the exhaust? If a cylinder (or, more) is not burning every so often, the fuel injectors are still spraying fuel. The fuel collects in the exhaust (perhaps, the catalytic converter) and gets ignited by the hot catalytic converter. (Where, from the engine to the exhaust, does the backfire sound like it occurring?)
Why doesn’t the fuel burn in the cylinder? Someone said sticking valves. Could be.
If spark isn’t happening, the raw fuel will go into the exhaust. Why wouldn’t spark happen? Sticking exhaust(?) valves? Fuel spray too rich, or lean? Fault in ignition system (primary, or secondary)?
OK, everybody, input?
Are the Damsels all better?
Maybe They "Threw In The Hat (Towel) Again At Beckley’s Liquors …
… or they’re holed up with the Donner Party! Could it be my automotive breath?
The missing third element that has not been discussed is air. Since Jeeps are a bit primitive has anyone checked the large air feeder hoses for problems. I once had an Alfa that experienced this type of problem. It was traced to an intermittent collape (due to separation) of the interior liner of the hose, especially when heavy acceleration and/or load was asked of the engine. Just a thought.
An Update from the Dames:
I’m sorry it’s taken me so long to get back with an update, but I had a big tune-up done in December/January and the jeep ran okay on several trips until the beginning of April when it bogged down between DC and Charleston, SC… this time there were 3 damsels in distress as I had a friend in tow on the way to the Cooper River Bridge Run. I had been running along at 60 mph for about 4-5 hours when it started the whole routine of choking out (air or fuel not reaching the engine), bogging down, and backfiring… my friend and I made it to Chucktown and back to DC via rental car while the jeep spent a week at a jeep dealership on the VA/NC border near where it broke down, but they could never get it to act up or find anything wrong with her despite numerous tests.
I got the jeep back and she ran fine until I drove her from DC to Asheville at the beginning of this month, so I switched cars with my dad. And just yesterday the jeep acted up for him and he thinks it’s vaporlock. Any guesses as to what would cause the vaporlock or how to get rid of it? I’m supposed to start a drive out to CO in about 10 days to start a summer job out there on June 7th and I’d LOVE to solve this problem so I can take the jeep (versus taking off in good old dad’s car & leaving him with plagued transportation for the summer as I fear that would jeopardize my title of “daughter of the year”).
Here are the parts I had replaced back in Dec/Jan: air filter, spark plugs, distributor cap, distributor rotor, fuel filter, idler pulley, left engine mount, and transmission output shaft seal. I’ve also had fuel injection service and oil changes done and the control head for my heater replaced. And before that I had my serpentine belt replaced, right rear axle seal & bearing and rear brakes replaced and my brake drums resurfaced.
A couple folks mentioned to me that since I have an older car it could be the increase in ethanol in unleaded fuel causing my problem so I’ve been trying to pump a higher grade of fuel and watch the ethanol content of the fuel I use (though often there isn’t a lot of choice). Some other folks mentioned that my model jeep has a second fuel filter in the tank and that could need changing… does either of those sound like they could be causing the kind of problem I’m experiencing???
Summary: On the downside, I still don’t know exactly what’s going on… but on the upside, I have been fully utilizing my AAA membership! 
However, I don’t have enough free towing miles left to get me anywhere close to Colorado so hopefully I can figure this issue out sooner than later!
Many thanks in advance for your input!!!
~Bogged-Down/Colorado-Bound
I think you need to seriously look at the fuel pump. Vapor lock is almost unheard of in fuel injected vehicles, but insufficient fuel pressure due to a weak/failing pump or a clogged filter are common. Also, did you try loosening the gas cap when the problem was occurring? That’s another more or less common kind of problem.
Oh, and by the way, alcohol in the fuel is not causing this. Don’t listen to the people telling you that. If the car were running right the 10% alcohol would not bother it, might decrease your fuel mileage, but it should run fine.
I once had a car that acted like this and the problem turned out to be very strange. It had a bad muffler that apparently internal pieces would shift around and partially block the exhaust. So it would not accelerate and would sputter, then after stopping and cooling off it would be OK again for a random amount of time. I’m not saying this is your problem, just that some times odd things can happen and they are hard to find. Hang in there . . .
I agree with Ranck on all his points, especially about checking the fuel pump…and the fuel pressure regulator.
Vapor lock is pretty much a thing of the past. Modern fuel delivery systems maintain high pressures (typically 40 PSI or more) of pure fuel all the way from the low point in the tank from which the fuel is pumped to the tip of the injector. There’s no low pressure spots or areas with air (like float bowls had) to provide an easy opportunity for boiling and vaporization to occur. Old fuel systems had the pump on the engine and pulled the fuel from the tank, so that line was very low pressure, and the way carbs work pulling fuel from what’s basically a bowl the level of which is simply maintained via the float and needle valve arrangement, those lines were low pressure also. Low pressure fluids vaporize under heat far more readily, especially if they have room to expand and stay low pressure.
I’d also want to check into the EVAP system. If it’s having diffiulty breathing in the tank can build up a vacuum on long drives and make it difficult for the pump to maintain pressure. This can ce hard to duplicate at the shop because as it sits the vacuum disspates and it runs fine again.
Have you been smelling any gas under the hood? That may be a sign of a charcoall canister saturation, and that might cause your fuel starvation after a long drive problem.
I agree with what’s been said so far…I doubt it’s vapor lock. Fuel injected engines are not really subject to vapor locking.
Sounds like a clogged fuel line…or bad pump. I’d bet money it’s fuel starvation.
I am going to stick with my original guess (from last year) that this is a valvetrain problem (i.e. sticking valve). The most likely cause would have been fuel delivery (bad pump), but that should have failed completely by now if that was the problem. The number of miles on this vehicle also make this sort of problem much more likely.
There are (of course) other possibilities, but they are less likely. No check engine light, right?
This problem does sound like a fuel starvation problem but I think another possiblity may be a bad crank angle sensor. I think they were notorius for going bad on Jeeps back then.
Wow, well done, Ranck!
I brought my jeep and a print-out of y’alls wonderful input to a mechanic and we drove around on I-85 until it acted for him… his impression was that the engine was being smothered and that it was an exhaust issue… he at first thought it was my catalytic converter, but turns out my jeep doesn’t have one of those. However, once he started checking into things, he took the encasement/cover off the muffler and discovered it was in pieces and the baffling or baffle (sp?) was periodically blocking the flow creating excessive back pressure at times…
They replaced her muffler and she hasn’t acted up once since then (though it’s only been 2 days), but she’s acting so much more peppy I really think the mystery has been solved at last!
Hopefully, she’s feeling peppy enough to get me and my sister from South Cakalacky out to Telluride, Colorado next week! Apparently a nickname for that city is “To-Hell-U-Ride” so we’ll see!
I thank each of you for taking the time to pass along your expert advise and input.
It was a HUGE help! It’s great to have a resource like this to turn to, especially when you’re not sure what’s going on and you’re no expert yourself. I tried posting on some jeep sites and I got some rude & downright perverse responses from some pottymouthed crazyheads so I was so happy and relieved to have this network to tap into & ask questions. Thanks again!
Cheers,
Damsels in Delight