I just bought a 1988 Bronco XLT 4x4 with the 302 and the AOD trans. I drove it around town for a day and it ran really well, but when I drove it 15 miles to where I work I ran into problems. First, I noticed that at a steady speed of 50mph intermittently I was getting a check engine light, and it felt like she wasn't all there at the pedal when that light came on. I pulled over at a gas station, put gas in it and checked my fluids, and I was about a half a quart low on oil. There was nothing on the ground under the truck, so I bought a quart of oil and left for work. Then when I came to a stop at a red light with the truck in gear the oil pressure gauge dropped and I got red light that said 'ENGINE'. When I gave it gas again and took off, the oil pressure gauge came back up, and the light went away. But every time I came to a stop the same thing happened. When I went on break I took my supervisor on a test drive b/c he used to have a bronco and is pretty good with cars. We drove it down the street and the first time I came to a stop the oil pressure gauge dropped to nothing, the red engine light came on, and the truck stalled and would not start again. There were no fluids on the ground under the truck. We dragged it back to our lot with a chain, and I was able to get it started so I backed it into a spot and parked it. I have no clue what's wrong with it so I thought I would ask the forums. Any and all help would be greatly appreciated as I have $10,000 invested in my two trucks and right now neither of them drive down the road. So this is my nightmare, give me your input.
Thanks guys,
Art

 

I experience the same problem in my 89' with a 302/AOD Bronco. It only does it after about 45 minutes of driving when the motor is good and warm and when it's about due for an oil change. I've been told the oil pump pick up screen is blocked, that there is a flapper door in the oil pan that is sticking preventing oil from getting to the pump, weak oil pump, bad pressure sender and bad gauge.

The best answer I have yet to find is worn cam bearings is the culprit for the fluctuating oil pressure. I've temporarily solved my problem with two bottles of Lucas Oul Treatment with every oil change. When the needle starts jumping around at around 4k miles I change the oil again

 

Ok guys, time for an update. So the bronco sat in the parking lot for 3 days while I tried to get it running. I was losing a lot of oil (a quart and a half in 20 miles). But I couldn't find any oil on the ground. That's not why it wouldn't start though. We thought it was a fuel issue so I put a new fuel pump in it in the parking lot. It started, but I put it in gear. tried to drive it, and she stalled after 10 yards. I hated to do it, but I called a flatbed and had her brought here. After we installed a new cap and rotor we were able to get it running yesterday before we started work. It ran for 10 minutes straight while we had the computer on it. We have been working on it nonstop from dawn to dusk for two days. So here's what I've found out. Blown out valve cover gaskets on both sides, leaking onto the manifolds so it was evaporating before it could run to the ground. So we replaced the gaskets. Also, we replaced the cap, rotor, plugs, wires, coil, pcv valve, fuel filter, fuel pump, oil sending unit, and fuel pump relay. This morning after we finished hooking everything back up, I started it and it ran for a couple minutes, and then shut off. We were not getting power to the fuel pump. The brown wire at the relay has no power. We kind of bypassed it with a piece of wire to get power to the fuel pump. Now we have 40 psi of fuel pressure at the rail with the key on, but the truck still won't start. If we spray carb cleaner in there it will start, and if we keep spraying it it will run but as soon as we stop she dies. We thought for sure that replacing the relays would fix it, but to no avail. My truck still won't start, and I'm out of ideas. Please help.

 

Well, it's fuel pressure for sure. Is there a fuel pump in the tank? The fuel pressure should be closer to 50 than to 40.

I use straight 30W in my 1990 Bronco, seems to like it.

 

I believe There is also a fuel pump in the tank.
It is 40 with the key on engine off, the book says that is well within the correct fuel pressure range.

 

Updating again as I have just now stopped working on her for today. We tested the injectors and they are NOT pulsing. Replaced distributor module/ignition module on the side of the distributor. No dice. I've ripped the computer out. It's sitting on the kitchen table, it looks like it has water damage; there is corrosion all over one side of it. I've ordered another one it should be here tomorrow. I've got my fingers crossed because after this I am completely out of ideas for this darn thing. Still want to hear what you guys think, please reply.
Thanks,
Art.

 

40psi is good fuel pressure for sure. sounds like your on the right track regarding the ecu.

 

So I put the new computer in today. Hit the key and she fired up! Ran like a champ for two minutes and then just quit. Didn't sputter or struggle just stopped. And would not start again.

 

If there is a fuel pump in the tank as well as 1 on the frame, The 1 in the tank does the majority of the pumping on 87-91 trucks. The 1 on the frame is more or less a "booster". I had basically the same problem with an 89 I used to own. Replaced the pump in tank and it fixed it. Dont know if it will fix yours or not though.

 

also that year bronco the oil sender isnt more than an idiot light at best and not a real gauge. I recommend you installing a real oil pressure gauge so you know whats going on. Also be sure that your not running a Fram oil filter. run purolator, wix, motorcraft.

what grade oil are you running?

 

I haven't posted in a while, so I'm going to update again. I've been going through this truck and replacing lots of things. So the reason the truck wouldn't run was a bad ground. I had it running and was going around checking the wires, I wiggled one and the truck shut off. So I fixed the ground and then it would stay running but was running really rough. I put a set of fairly new fuel injectors in it that came out of a 351. I made sure they all sprayed. While I had it apart I replaced the coolant temp sensor and the map sensor, and when I put it back together, the truck started with a shot of carb clean and stayed running and sounded pretty good. So I've been driving it to work every day since, but still having problems. So here's what I'm experiencing. First of all, the truck won't start when it's cold without a shot of carb clean. Then it starts, but it runs rough and smokes out of the tailpipes for up to five minutes. Then the motor jumps up to like 1800 rpms, and the truck smooths out and stops smoking out of the exhaust, but still misses occasionally on both sides. Earlier this week I checked all my fluids and I was a quart low on oil, so I added a quart, friday I had to add another half a quart. The truck seemed like it was having issues on friday because the miss that I'm having became much more frequent, and it's on both sides. So I parked it. I compression tested all the cylinders, and they were all between 165 and 180 psi. None of the spark plugs have oil on them I have put 500 miles on them. Oh yeah, I went out and bought a mechanical oil pressure gauge, and installed it. When the engine is hot, the mechanical guage shows 25 psi when i hold it at about 2500 rpm, then when I let it drop down to idle, the oil pressure drops to nothing. I'm curious what you guys think, particularly about it not starting cold, why it would miss like it does, and about the oil pressure issue. Thank you.

 

your main bearings are worn. what oil are you using and what brand oil filter?

oil is going somewhere, and with it smoking like that i am suprised you got those numbers on the compression test. you might replace the pcv valve and also move the pvc line to the top of the intake vacume tree. you can move the brake booster vacum line to the location on back of intake where the pvc was originally plumbed to.

did you notice any oil when you pulled the upper intake? how did the runners look?
whats your timing set at? you know to pull the spout connector first. and have you replaced the cap/rotor.

 

The guy I bought it from said it had been sitting, so I did an oil change I put Mobil 1 10w-30 and a fram tough guard filter with the intent of changing the oil again in a month. I replaced the pcv valve and the hose when I had the upper intake off. Did not notice any oil and everything that I looked at looked good. I set the timing on Saturday it should be good. Brand new cap, rotor, distributor, coil, plugs and belden wires.

 

Fram filters are known to cause oil pressure issues, you can also try moving over to a Rotella or Delo 15w40 which is a good high detergent oil that will really clean up the inside and has the zinc which is important.

you running standard copper plugs? Ive never heard of Belden wires?

regarding the pcv, there is a ford TSB regarding the line needing moved to the center of the intake.

 

Everyone I've talked to keeps telling me to use 20W-50 in it and run a good filter like purolator or motorcraft.

Running brand new Autolite 25's. Belden wires are the Napa Lifetime Warranty wires, I've always had really good luck with them.

Where would I find the Ford TSB referencing the PCV line?