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Need Help From Someone Who's Smart and Knows Motors

This is a really tough one. So, I have brought it to the experts.

Here is the data:

- Recently replaced the head gaskets on a 3VZE (v6 in '88 4Runner) - Lapped the valves, replaced all the seals, cleaned everything up really good - New belts, gaskets, injector bushings and o-rings, idler pulleys, oil pump, sending unit, water pump, etc, etc. - Adjusted the valves - Put it all back together and the engine runs like a cylinder is missing - Pulled plug wires off of distributor cap one at a time and, bingo, No.

6 makes no difference in the engine running rough. Pulling other wires off makes it run even rougher. - Plugs are new, cap is new, rotor button is new. But, check plug anyway. Replace plug anyway. No difference. Engine still acts like cylinder is missing. - Check compression. It's okay at 145 psig. Not bad for 300,000 miles. - Other cylinders have similar compression - Pulling wiring harness connection off No. 6 injector makes no difference in the way engine runs either. Again, it's like the cylinder is not firing so this is not surprising. - Resistance is okay on injector and little voice says, "it's not the injector, stupid" But, replace injector anyway. Engine runs maybe a tad better, maybe not. Essentially no difference. Still feels like No. 6 cylinder is not firing. - Notice that pulling connector off injector at idle now makes a very slight, but noticable difference. The eingine speeds up a just a hair when the injector is disconnected. Strange, but not really - it would take a tad more energy to suck gas-laden air into the cylinder than just air alone. So, engine running a tad faster with injector disconnected may make sense. - Timing is good. Coil is good. RPMs are correct. Really, there isn't much left other than taking the engine apart again and seeing what I can find which will probably be nothing because I just had the thing apart.

What could this be? I am racking my brain and coming up empty. If a valve was sticking, I wouldn't get good compression, correct? Besides, I just cleaned, lapped and put new stem seals in for the valves.

I am at a total loss. If someone can solve this one for me, I will be very impressed, highly in debt and may have to send you a gift basket. Be warned, however. This one is only for those who work with motors and are relatively smart. It's not for the novices nor the mentally challenged.

Everything points to No. 6 cylinder. Could I have messed up the valve adjustment? I tweaked about 11 out of 16 valves during the adjustment and these two didn't get any more than the rest. I tightened both up just a bit to be back in tolerance (went from factory to #19 and #17 shims.)

Or, could the No. 6 cylinder thing be a red herring? Is there something else that could be wrong that would only show up in that cylinder? I mean there is NO difference in how the engine idles when that plug wire is pulled off. There is a BIG difference when any other is pulled off.

One last datum point. It seems to run fairly well at speed. I think the cylinder is still 'missing;' it's just not as noticeable. But, it runs like heck at idle.

I need help on this. I have been very careful to put this thing back together in good fashion, but I apparently buggered something up.

TIA to the genius that figures this out.

Luther

Reply to
Luther
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New plug wires? If so, have you switched a wire from another cylinder with the one on # 6 and tried that?

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Reply to
BBTACO

Reply to
Baffman

Let me guess. Your #6 injector did not get electrical power signal either the wiring hardness get shorted or broken. To a greater extend, maybe the IC that drive this injector got burned out.

Reply to
DTT

Did you check #6 plug wire resistance? You may have a broken wire.

de ac4xe

Reply to
Jon Waring

I'm betting on a vacuum leak on the intake manifold for #6 cylinder. This can be tested a couple of ways.

1) Spray carb cleaner around the manifold with the engine idling. The idle should increase when you spray near the leak. With an intake manifold leak, the mixture will be leaned out (too much air for the fuel). at idle, the cylinder will misfire or maybe not fire at all. At higher speed the leak is not significant compared to the amount of air/fuel being drawn into the cylinder, so the cylinder will probably fire normally (or nearly normally). The carb cleaner is combustible. When it is sucked into the intake at idle, it will provide a richer mixture and the cylinder will fire and the engine will run faster. 2) Spray water around the manifold with the engine idling. The idle should increase when you spray near the leak. Much the same, except this time you are plugging the leak more than injecting a richer mixture.

Good luck. if my guess is correct, this should be an easy fix.

Ken - West Union, SC Population 270 + 2 (counting dogs and chickens) "There are 2 theories to arguing with a woman...neither works." - Will Rodgers

Reply to
RunnerBiker

I have checked the plug with it out of the vehicle and grounded to the fender skirt and it sparks like a son-of-a-gun. I don't think it is the plug or wire.

Thanks,

Luther

Reply to
Luther

These motors use electronic ignition advance rather than a vacuum advance. I have had the distributor out recently - that is true. However, the definitive test is to set the crank at TDC and verify that the rotor is pointing at No. 1 plug wire (or 180° off). It is.

This could be the problem - but not likely as I have checked the timing very carefuly and this would affect all cylinders.

Thanks,

Luther

Reply to
Luther

Not a bad guess. This has not been definitively ruled out. The wiring harness is brittle on this vehicle and a short is possible.

I need to check this and I will let you know.

Luther

Reply to
Luther

Ignition has been all but ruled out, but I may have a look at the wire resistance anyway.

Thanks,

Luther

Reply to
Luther

Now we are getting somewhere. I have had the manifold off (replaced the gasket) and this would explain the symptoms.

I will check this out and let you know. It DOES feel like a vacuum leak in general - however, I didn't know it could leak at just one cylinder.

The only thing that I could think of at this point is maybe I dropped a screwdriver down No. 6 intake on the manifold and didnt' notice it. That is how it acts. Wish I had looked closer when I swapped the injector.

Thanks for the tips and the diagnostics. I will definitely give this a try.

Luther

Reply to
Luther

Reply to
pck

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