Driving me crazy or at least close.

[91BroncoToy]

OK...OK....OK. I have a delima that is driving me.....crazy. 91 Bronco E/B 5.8 EFI, E40D. Taking off from a dead stop, engine runs fine, transmission shifts into 2nd gear. Engine rpm normal range. Transmission shifts into 3rd, engine runs in normal rpm range. Then it's as if the engine is running out of fuel. I had a vacuum gauge on the intake system and at idle the vacuum is 20 inches. The vacuum drops to zero during acceleration, but as the engine gains power the vacuum returns to the 20" range as if it's a straight road with no load on the engine. It is as if the transmission is shifting into overdrive too soon. I turn off the overdrive because there is a switch to do so, and it doesn't make any difference. I have checked the TPS and it checked out fine. I thought maybe it was the MAF sensor, so I swapped it out with a known good one. No difference. I unplugged the EGR position sensor, no difference. I ran with the transmission in first in first gear, no lack of power. I did the same thing in 2nd gear, no problem. I also thought the transmission may have been slipping, but the fluid is bright and clear with no smell of being burnt. I even went so far as swapping the distributor because I was thinking maybe the pick up was failing. I get no check engine light. I checked codes yesterday and it showed codes 21 for engine coolant out of range, which I know what caused that. I got a code 26 which is MAF or VAF out of range, so I changed out the MAF, and code 24 intake air temp or vane air flow out of range, which I am a little fuzzy on what to do about that one. Any help or direction would be greatly appreciated. Yes I do love my Bronco. I just picked up another one and my Son and I will be making an off road toy out of that one.

[greystreak92]

How do you have a MAF sensor on a 91? MAF wasn't even available on the Bronco until 1996. Better yet, which sensor DID you replace? Believe me, I'm not trying to be antagonistic here. I just worry that you spent money to replace something that didn't need it.

At any rate, Code 21 rears it ugly head when the KOEO and KOER tests are administered without warming the engine to normal operating temperature first. KOEO test requires engine temp. to be above 50*F and not above 250*F. The KOER test requires engine temp to be above 180*F and not above 250*F. If you are certain the engine was within the proper range at the time the tests were administered, the next and ONLY step is to make sure you have VREF (+5VDC) at the ECT sensor with the key on. The exact same situation and troubleshooting goes along with Code 24 which is the exact same indicator for the only other thermal sensor on the engine... the ACT (Air Charge Temperature) sensor.

Code 26 in 5.8L trucks with the E4OD (yes its that specific) has absolutely NOTHING to do with anything but an overheated transmission. Code 26 in your truck means that at some point during the last 40 operations of the engine the TOT (Transmission Oil Temperature) sensor got a reading that was too high... or too low which is about as likely as winning a trip to Mars next week. My guess would be that when that happened, you had some transmission damage occur. Then again, you might want to clear the codes and see if it happens again. Any code related to the E4OD transmission sends the computer into a special mode that keeps the engine from doing anything to jeopardize the functionality of the transmission. It reconfigures shift points, keeps it from down shifting when goosed etc. Clearing the memory may put things back to rights. Just be aware that should the TOT sensor get this reading again and you pull Code 26 again, you might want to have the pros look at the E4OD.

[91BroncoToy]

I hit the wrong key and meant MAP sensor. Code 21 was when The engine temp went out of range when the heater core was clogged and the engine was air bound until it was repaired and filled up. The transmission overheating thing is what I am going to lean towards but I am not sure when this could have happened. I will pull the battery cable off this morning and put it back on to clear the codes and we'll go from there. I appreciate your time.

[helirich]

A few months ago my truck felt like it was running out of gas in high gear. Changed the fuel pumps and filter and it was cured. Just a thought.

[91BroncoToy]

I change the fuel filter. No change. Would a 02 sensor cause these issues?

[Conanski]

Check fuel pressure, it should be about 35psi at idle and it should increase to 40-42psi when the throttle is blipped or vacuum removed from the FPR. If the pressure is below this level or drops without vacuum the fuel pump is going.

[91BroncoToy]

I will check the fuel pressure too. I think what really annoys me is that it doesn't happen all the time. I was thinking that if the fuel pump was going, it would happen all the time not intermittent. Is the fuel regulator is vacuum controlled? When I put a vacuum gauge on the intake manifold, I am getting 18 - 20 inches of vacuum. To me, that's a good sign of a healthy engine. When I hit the throttle it drops and as soon as you let the throttle go, it goes right back to the good range. I was out checking something else and noticed that when I feather the throttle and watch the vacuum gauge, the gauge is very responsive. I think I will check out the 02 sensor next. I got another Bronco and there are parts on it that I can use on this one. The one I bought had an under hood fire which the person I got it from said it happened after being parked for 15 minutes. The heater area was toast. My Son & I have decided to make an off road vehicle out of it. Should be fun.

[helirich]

My problem with the fuel pump was intermitton. Although it mainly accured when at high demand. (going up hill on the highway)