rough idle mystery


Asked by joeyWV6Q5 Apr 27, 2018 at 03:44 PM about the 1981 Volkswagen Pickup FWD

Question type: Maintenance & Repair

i have a 1981 vw rabbit pickup with a 1.7 gas motor. bought
it a few months ago from a guu who had it sitting in his
barn for the padt 6 years. it wasnt running when i bought
but after many weekends i finnally got it running. my
problem now is it was a rough idle where it appears
cylinders 1 and 4 are misfiring maybe. ive replaced spark
plugs, fuel filter, cap, rotor, plug wires air filter, injector seals
ect. the works. it drives great but idle is very rough. i
decided to take it to a shop to have a proffesional loom into
the deal and this is what he did: repaired vacuum leaks,
balve adjustment, checked timing and everything checked
out good. he did a compression test, the results were: cyl.
1: 170 2:150 3:150 4:170 and a leak down test and the
results were cyl. 1: 0-40% 2: 20% 3: 30% 4: 10-50%.  so
basically the two cylinders with the best compression had
the worst leak down. and those are the cylinders that
appear to miss at idle. the strangest part is when you pull
the wire on either 1 or 4 the idle smooths out and idle
increases where if you pull 2 or 3 idle drops like its
supposed too. what would cause idle to increase and
engine to smooth out when you disconnect th cylinder? and
yes the firing order is correct. the mechanic i took it to said
he didnt know ehat would cause that, the only thing he
could think of would it might need a valve job but couldnt
tell me for sure. any ideas??

6 Answers


I don't have time to read the entire story, but if it has been sitting for quite some time, it's probably out of tune. I would change all of my fluids. Tires are probably weak, too, since they have been sitting. Junk yards have new tires for just a few bucks a piece. As for your bad idle, the first thing I usually do when I get a car, is change my plug wires and spark plugs. Your plugs are either out of gap or were not gapped properly to begin with. good luck

The fuel system is probably fouled with old, oxidized gas. Flush it out and use some good fuel system cleaner.


thanks for the responses. all new fluids, new tires plugs and wires have been changed and mechanic did an intake flush and tuned the motor the best he could. he also checked the spray pattern on the injectors and said they were decent. running some premium gas through it now and some injector cleaner with no improvement yet

1 people found this helpful.

found my problem. there were multiple vacuum leaks. only reason i didnt check before was because the mechanic i took it to said he took care off all vacuum leaks. two cans of carb cleaner later i found them, one being the egr valve gasket and connection to manifold. smoother than silk now

2 people found this helpful.

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