What repair fixes a misfiring 4-Runner?

January 12, 2009 8:42 AM

Why does my uncle's 1998 V6 Toyota 4-Runner misfire at highway speeds? Last month we drove from Los Angeles to Tucson and back, and at what seemed to be random points while we were driving the engine would buck repeatedly. Once it started, it would continue at all speeds, with the engine running particularly rough at low/idle RPM. We took it in to 3 different Toyota dealerships along the way, and to it's home dealership once we got back. They were all happy to service the vehicle, but none were able to fix it and two insisted they couldn't find any problems with the vehicle.

Before we go on, my uncle has always insisted on dealership service. His 4-Runner is in pristine condition, to the day on all of its tune-ups and servicings, and only has 130,000 or so miles on it.

The problem first started near El Centro. We limped south along hwy 86 to the dealership there, where they diagnosed a misfire on 'cylinder one'. They 'made some adjustments' but didn't replace or repair anything, and as we left the truck ran fine. In fact, it ran great all the way through Yuma and Casa Grande, but by the time we were passing Picacho (just north of Tucson) the problem had started again. Throughout this leg of the trip we didn't see any instrumental indication that the engine had a problem. (No check-engine light.)

Then Christmas came, and all the dealership service bays were closed. Eventually we got it into a dealership in Tucson, where they couldn't find a whit wrong with the vehicle. Since it was due up, my uncle went ahead and sprung for a full tune-up while we were there. He didn't test the truck at highway speeds after getting it back though...

And sure enough, a few days later as we were rounding Gila Bend on the way back to LA, cruisin' along at a stately 70mph, the engine started bucking again. The check engine light came on occasionally. This time we limped further, all the way to Yuma, where the young salesschmuck told us after 3 hours of diagnostics that they could start by replacing 'the coil', that the distributor needed replacing as well but since they were in the middle of nowhere they didn't have the part on hand. Fine, said unc, so they replaced 'the coil' and sent us on our way. We weren't 20 miles outside of Yuma before the engine started misfiring again.

We make it home (14 hours, Tucson to LA) and my uncle takes his truck into its home dealership, where it's been serviced since he bought it new. According to him, they can't find a single thing wrong with it, including the distributor. There's nothing they could do, apparently.

So what the hell? What could be causing the misfire? Why won't any mechanics diagnose the problem? What are they, what are WE missing? My poor uncle has poured thousands of dollars into repairing his beloved vehicle, and it's still not fixed. I'd love to be able to help him get it running right again.

posted by carsonb to travel & transportation (10 comments total)

This may help.
posted by weapons-grade pandemonium at 9:15 AM on January 12, 2009

weapons-grade pandemonium likely nailed it -- Mass Air Flow Sensor.
posted by M.C. Lo-Carb! at 9:34 AM on January 12, 2009

I had this problem once and it was misdiagnosed by a multitude of mechanics, including a couple at dealerships. The culprit turned out to be a bare wire feeding the alternator which at high speeds shorted out against ground. The wire was inside of a plastic tube so it wasn't obvious from external examination that it was bare and finding it was tricky, especially as it was not shorted when the car wasn't moving. It was one of those wire protecting tubes with a slit along its length so it can be placed over wires without disconnecting them. I think the bare wire was blowing out of the slit at high speeds. This is probably not your problem, but it is something to look for.
posted by caddis at 9:46 AM on January 12, 2009

Some good suggestions, but let me add bad spark plug wires to the list. Speaking from experience here ('91 Toyota Pickup v6 with similar symptoms)...
posted by jluce50 at 10:31 AM on January 12, 2009

The fact that it happens it happens after you've been driving for awhile means something is changing either (a) at speed, or (b) at temperature. I'm not super familiar with Toyatas, but I had a Honda that, when it got warm, things would expand and the seals meant to keep oil out of the spark-plug chambers would leak. Oil would get in there stop the plug from firing. New seals, plugs and wires solved the problem.

Since this is a spark issue, start with the plugs and work backward until you find the problem.
posted by jeffamaphone at 11:58 AM on January 12, 2009

try loosening the gas cap when it happens and see if it quits. Gas cap may not be venting properly. After running awhile, enough vacuum in the tank that the fuel pump can't pump properly.

Easy to test, cheap to fix.
posted by Dorri732 at 1:58 PM on January 12, 2009

Make any enemies lately? Check for contaminants in fuel supply. Water, sugar, oil etc.
posted by Muirwylde at 4:28 PM on January 12, 2009

This is the frustrating thing about this problem..it could be all of the above or none of the above. I had the exact same problem with a Saab. I think I may have even posted an askme about it. I tried pretty much everything. Not a single mechanic could fix it. So I sold the car. Kid who bought it fixed it in five minutes...it was a worn hose into the engine that was coming loose and causing the engine to lose pressure. He taped the hose and voila..no problems. So, I would start with a thorough survey of all wires/hoses/other connections both electric, air, gas, etc and make sure everything is snug, sealed, insulated, etc.
posted by spicynuts at 8:45 AM on January 13, 2009

Have them check the TPS, that can cause similar problems (throttle position sensor - at the other end of the throttle cable to your foot).
posted by GeeEmm at 11:25 PM on January 13, 2009

Thank you all for bringing these possibilities to light. The array of possible causes is daunting, to say the least. Would any of them have been covered by the complete tune-up my uncle purchased while in Tucson? For instance, I believe he told me that they replaced his spark plugs; would they have checked the spark plug wires over the course of that replacement?
posted by carsonb at 9:20 AM on January 14, 2009