Questions and Answers
4 Runner With A Long Story
Q. Well maintained 1991 Toyota 4 Runner SR5, V-6 3.0 liter 3VZE engine, 5 speed Manual transmission, 172,000 miles, Fuel Injection, Rear ABS brakes, P/S - A/C - Cruise control with a bucking bronco of a problem.
About six weeks ago my 1991 4 Runner V-6 4WD began to lose power after driving on the freeway or at higher speeds on surface roads for a few minutes. I can only describe it as a "lugging" sensation since the whole vehicle would begin to shudder as if I was trying to go up a steep hill in third gear from a dead stop. This would occur at 2500+ RPM's and would be temporarily (i.e. momentarily) relieved if I shifted down and revved the car to 3500+ RPM's. It was also more severe with the AC on but it would happen with it off as well.
One key observation was that it would not occur if the engine was cold. I had to warm it up for at least 10 minutes to get this symptom.
I looked under the hood and found a cracked distributor cap and a very oxidized wire leading in from the coil. Since it was at 171,000+ miles and was due for a tune-up anyway I brought my car to my newly recommended mechanic and had them do a complete tune-up including new cap, rotor, cables and plugs. They also changed the oil and replaced air, oil, & fuel filters. I drove home three blocks and it felt great and parked it for the night.
The next morning the car was running great! More power than ever! That is until it warmed up (this time on the freeway) and began to buck and shudder as if it had bovine ancestry! Pulling off the freeway I opened the hood and listened to the engine. It was idling low at ~500 RPM's and every once in a while almost "dying." If I revved the engine the idle would increase to about 850 and then slowly go back down again until the engine cooled off (with the auxiliary fan--note that the temperature gauge on the dash reads normal).
I brought the vehicle back and asked the mechanics to take it on a long-test drive to feel the actual problem and then diagnose the problem and get back to me.
On the test drive the owner felt that it was fuel system related. Well they connected it to a computer (yikes) and it showed the fuel pump as delivering 30+/- psi (I am not sure of their measurements or units) instead of "75" when the engine was warmed up. So they called me and I authorized them to replace the fuel pump. I had a niggling feeling that this wasn't right because why would the fuel pump performance be related to the operating TEMPERATURE of the engine (it sits in the gas tank doesn't it?) but I didn't say anything. That afternoon I shelled out more than 3 c notes and drove off.
To make a long story, well, longer... the next morning I drove about 10 minutes and my suspicions were justified. Buck and shudder all over again with the exact same symptoms! After calling my old mechanic (who implored me to get them to check the plug gap and plugs) and consulting my Haynes manual I drove back to the mechanics and got the owner to sit in the passenger seat and took him for a ride (heeheeehee) :)
I suggested that they focus on three possibilities: ignition, vacuum pressure and lastly the fuel system. My personal favorite hypothesis is that increase in temperature increases resistance in an ignition component (coil?) and that causes the missing /rough engine problem. But how would that relate to the fuel pump?
The same mechanics replaced the fuel pump again (no change in symptoms!/no charge!) even though I told them not too (their loss). Why would they do it twice unless the substituted a defective part to begin with. I brought it back AGAIN and then they replaced the TPS switch they said to drive if overnight and see if there was any improvement. That evening it ran OK but by morning I could tell that the TPS didn't help one bit was shuddering all the way to school (12 miles). By now I was frustrated and angry enough to not pay for the TPS (too bad I already paid for two new unnecessary fuel pumps (once)).
I checked the coil spark output using a simple spark voltage measuring device and it was very strong. No problem there!
I brought the car into the largest Toyota dealer on the island and they checked a few things and charged $40 to tell me that the Throttle Body needed replacement because it was passing too much air when hot. This sounded reasonable but I wasn't about to pay $700 + labor for them to do it so I ordered a used one from a junkyard and got it shipped down from Washington. I cleaned this one out with carburetor cleaner and moved the new TPS switch from the old to the replacement throttle body and then set the TPS switch as per my manual and then installed the throttle body. The car once again ran great.this time for about 2 days. Until it started to shudder again! I tried out various diagnostics to understand what Toyota was telling me about the throttle body and found that the car failed the "air valve" tests (no change when you screw in the idle when warm is OK, while mine still drops when warm indicating the air vale is stuck open). I spent about 2 hours looking for the air valve on the 1991 3VZE engine and then gave up and asked the Toyota guys where it was. They didn't have a clue until they looked in their manual and indicated that it was an integral part of the throttle body! No wonder they wanted me to buy a new one!
Another symptom of the "new" version of the old problem was that the idle was too high with the idle screw screwed down all the way. This happened when I installed the replacement used throttle body. Also on the "new/used" throttle body dashpot appeared to be out of adjustment so I readjusted this following procedure (I later found out this is supposedly a no-no). The last change gave me the best improvement in performance. It is now only a little "cranky" at ~2500 RPMS with no big shudder but a tiny bit of power fluctuation when I go from acceleration to deceleration (foot off of the gas). However, I am certain I have only generated a band-aid solution to the problem because my idle is off 95% of the time and it fluctuates down from high 1500's to about 1000 unless the original problem kicks in (with its new set of kinder, gentler symptoms).
Although the Toyota mechanic suspects that replacement throttle body was also defective I feel it is much more likely that in spite of the bad adjustment of the TPS and dashpot the ECU is just getting bad data from a sensor (possibly the 02 sensor?) or the EGR valve or other emissions biscuits are defective and throwing the system off.
So now I have two problems:
My questions are:
- Get rid of the fast/fluctuating -idle problem (presumably to set it back to the original condition before they messed with the TPS switch and I replaced the throttle body (also messing with TPS and dashpot in the process) and then.
- Solve the bucking engine problem (assuming I can get it back to the "original" fully bucking condition).
- Should I try to further clean the old throttle body air valve out by soaking it in throttle body cleaner overnight and then replace and readjust the TPS switch and re-install the old throttle body to try to "solve" the fast-idle problem and see if it was a stuck open air valve to begin with?
- And/or should I test the various sensor outputs and grounds to see if I have a "bad data" sent to the computer problem and then go from there?
My worry is that before the mechanics first messed with the TPS at least the cold idle and cold performance seemed completely normal. If I don't go back to this "state" I am not sure I'll ever be able to isolate the original problem!
Until I can find a better mechanic/diagnostician than myself here in Puerto Rico any suggestions would be greatly appreciated, as I have no fingernails left!
Thanks in advance,
DurrellA. WOW! I didn't know whether I should read this book you wrote or wait for the movie to come out. But that's okay, with a problem like this I can use all the information I can get. let's just hope I can do something with it.
First of all the fuel pressure should be:
FUEL PRESSURE:
At Idle 33 - 37 PSI At WOT 38 - 44 PSI Also just because a fuel pump is new, that doesn't mean it's good. I have installed new parts that were no good and had to replace them again.
If it were me trying to diagnose this car in my shop, the first thing I would have done is to hook up my scan tool and see if there are any codes stored in the computer. Then I would run a real time test to make sure all the inputs and outputs were correct. Nine times out of ten this will put me in the ball park. Then I would have done the fuel pressure test. I would not have replaced it with the readings they got however.
Just from the description of the symptoms, I would have checked the coolant temperature sensor first. When cold it will be fine but when warm it will cause the problem you are experiencing. Cleaning the throttle chamber and Idle Air Control Valve would be the next thing to do. Gum and varnish accumulate in there and keeps it from operating properly.
Another thing to check would be the PCM. Toyota came out with a new PCM for Trucks built after August 1991 with manual transmissions. The revised PCM improves the driveability under conditions similar to what you describe.
Additional Information provided courtesy of ALLDATA
