Bucking problem with 1986 Jetta GLI

My wife has a 1986 Jetta GLI that has been having a strange "bucking" problem the last year or so. Most of the time it runs fine, but when the weather get's warm the car will "buck" and "jerk" when put under load (i.e. trying climb a hill, or accelerate too fast). Otherwise, it idles nicely, cruises smoothly, passes emissions tests easily, and gets good gas mileage.

About the only other symptom I have seen is that it will sometimes do this short "hunting" when you come up to a stop sign. When you take your foot off the gas to hit the brake, the engine will accelerate just slightly before returning to idle. Again, this is hardly noticeable, and very infrequent.

We've done a complete tuneup with new air filter, fuel filter, spark plugs, cap, rotor, and plug wires. I replaced the Oxygen sensor a while back too and it didn't change anything either.

The tuneup helped briefly, as does putting in a higher octane gas, but neither makes the problem go away completely. And, when the weather cools down, the problem seems to go away.

Where to I go from here?

Anthony

Reply to
HerHusband
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Certainly check the air intake, though in my experience a crack in one of the boots makes it run worse at idle and low rpms. When you hit the gas, the vacuum compresses the boot and seals the crack. Perhaps it would act differently if one of the hose clamps is loose.

I'm surprised that the new ignition parts did not help, especially the new wires. It sounds to me like the engine is not firing properly when under load, which could be improper mixture (vacuum leak) or a weak or improperly timed spark. I'll assume your timing is good from the tune-up, though it wouldn't hurt to double check.

I think it is likely an ignition problem. Check your coil. If possible try a different one. Then test the ignition control unit. If those check out, it could be a faulty Hall sender. They sometimes seem to be a heat related problem. I think it's more likely that your coil acts up in the heat and you notice the weak spark only when under load.

Just my two cents...

Jason

Reply to
Jason Faas

Your excellent description of the symptons all sound like classic symptoms of the dreaded in-tank fuel transfer pump failure. This "pre-pump" feeds fuel from the tank to the main fuel pump which is housed within the fuel reservoir under the RR seat area.

The fuel reservoir has to be kept full of fuel to both keep the main fuel pump cool and to prevent air bubbles making into the fuel lines running to the engine compartment, and it's the transfer pump's job to keep the reservoir full to the brim. But when the in-tank transfer pump is running at or below minimum spec it will get worse on hot days and cannot keep the reservoir full which results in sporadic bucking when an airbubble gets sucked into the line. I always tell people to check this by driving up a long hill in a lower gear at high RPM to see if they can induce bucking, but you've already noted this is happening. If the reservoir isn't jam packed full of fuel, driving uphill will cause the fuel to move to the rear of the reservoir and drop below the main pump's fuel intake and you get the dreaded air bubble and subsequent violent bucking.

Another good clue is how the engine hunts when you come to a stop - this is because the fuel in the half-full reservoir gets sloshed front to rear and is causing slight cavitation of the main pump similar to when you're going uphill and is almost stalling the car then it catches again with every slosh. If you were to keep the right side windows down you'd probably hear the main pump buzzing loudly with every fuel slosh at the stopsign. Kinda sounds like this:

rrreeeee-uuurrrrrr-rreeeeee-uurrrrrrr-reeeeee-urrrrrrr :-)

Go to

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and click on Groups, then type or paste this phrase into the field:

Randy Walters Transfer Pump

You'll get tons of old threads about this common problem :-)

Also, virtually all MkII Golf/Jetta cars were covered under a recall to have the transfer pump replaced free. Both my old VWs qualified for the recall as did tons of other people on this newsgroup. Go to your friendly local VW dealer and have him run your VIN in his computer - you may find that yours has never been replaced and you should be able to get one installed free. Otherwise it's an easy job to do yourself (i've done several).

Randy

85 Golf GTi (retired, about to be sold) 86 Jetta GLi (sold to the Potterman)
Reply to
Randy Walters

Followup on my wife's Jetta...

It appears that the problem is finally fixed. It turned out to be two small, but unrelated problems.

The most significant was a broken ground wire on the intake manifold, which appears to run to the ignition control unit. I literally discovered this by accident. The wire was still sitting in the housing of the crimp connector, but apparently wasn't really connected. I bumped it with my hand when I was trying to check everything out, and it literally fell out of the connector. I'm guessing it would touch the connector and run fine most of the time, but as the engine would rock under load, it would lose the ground connection and start the bucking and jerking. Anyway, it was a quick and easy fix. I never would have found this just by looking! I've even tested those wires with an electrical meter in the past, but it was apparently touching at the time.

The second problem was a small crack in a vacuum hose that runs from the intake manifold to the idle stabilizer. Again, I made it worse when I was reaching around trying to find the problem, basically cracking the hose completely in half. But, my clumsiness made the problem easy to find and fix... Ha. Ha.

Hopefully this will be of some benefit to others experiencing similar problems in the future.

Anthony

Reply to
HerHusband

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