My 1988 Bronco with a 351w has been into two shops and is still not running properly. It idles around 1000 rpm. It stalls during deceleration. The tach gauge shows the rpm dropping towards 0 when running. The mechanics or I have replaced the fuel filter, EGR valve, coil, plugs, ignition module, rotor and cap, IAC, upper and lower intake manifold gaskets, fuel pressure regulator, 3 slightly leaking fuel injectors, O2 sensor, MAP sensor. The #3 cylinder fouls the plug instantly. However, compression testing shows all cylinders are fine. Black carbon is coming from the tail pipe. The engine must be running rich , but why?? Codes have been pulled and nothing stands out as to what is causing this rough running. The last time we had it out for a test drive, the red engine light came on and the oil pressure drop-off to zero. Towed it back, waited a couple of days and started it back up. Oil pressure returned. I have been considering replacing the motor, but not I am not sure problem would be solved and I don't want to ruin the replacement motor. There have been a couple of weeks where the darn thing would run just fine and then run bad for a day and then run fine for a couple more weeks.

 

TPS, maybe? Might be the computer itself. Or maybe a bad ground to the computer. I think they can screw up and not throw codes. Couple of shots in the dark for ya. Good luck.
Oh, and welcome to FTE!

 

i would say computer as well.and don,t be put of by the 2 shops,nobody knows how to work on this stuff.

 

I had a problem like that on my 88 F250 300 six all it turned out to be was the wire going into to the computer was corroded. I were caseing that problem for well over 6 months. Give that a shot and keep us posted. Good luck.

 

Are you kidding me? FUEL PRESSURE REGULATOR!

 

He said he already changed that...

 

OOps I missed that. I would still have them check the fuel pressure and make sure the new one is good. I've had this happen to me before and a lot of friends and it's always the FPR.

 

So what were the codes?

Lots do, you just haven't found them.

Too much fuel - could be stuck open injectors, too much fuel pressure, damaged fuel pressure regulator (already replaced), bad MAP (already replaced), damaged wiring, and probably some stuff I am forgetting. From your description, it sounds intermittent, which in many cases is wiring. Sit down with a wiring diagram and check MAP wiring, and fuel injector wiring to make sure nothing is shorting out, either to ground or power - and check for continuity in the that wiring as well.

 

the point i was trying to make is if they can build this stuff,they should be able to fix it without changing half the motor.you go in with a little miss or stall and you come out with a thousand dollar bill.and that,s at the dealer.

 

Yeah, that's the way they work a lot of times, sadly.

 

The most recent codes are 34 (high exhaust pressure) and 53 (throttle position sensor open/high). I replaced the TPS yesterday morning. Upon removing the throttle body, the plenum side of the fins had a coating of black carbon (moist). I had the throttle body off before and it was relatively clean. The plenum was also lined with carbon debris. I think the constantly running rich condition is starting to magnify the initial problem. I pulled the plug from the #3 cylinder and it is carbon fouled and smells like gas, but not wet. The #1, #3, and #4 fuel injectors were the ones replaced.
Today, I am looking for a wiring fault. The first shop replaced the O2 sensor and located the fouled plug on #3 cylinder. First compression test was done at this shop. They recommend replacing the manifold gaskets. The second shop replaced the EGR valve and pipe, coil, plugs, wires, reran compression test, replaced intake gaskets, cleaned all electrical connectors. The Bronco continued with the intermittently fine one day bad the next scenario. It now only runs badly. It wll run fine for a few seconds then the tach will drop to zero followed by a return to high rpm and then drop back instantly and then back up again. I think fuel or wiring is/was the first problem.
I am focusing on two theories for the #3 cylinder 1. the cylinder is not combusting all of the fuel. or cylinder is getting more fuel than it can compress. I plan to pull the valve cover off this week.

 

Focus on the TPS problem. The voltage at closed throttle should be ~.9V and then at full throttle, it should be around 4.8V. In between, it should smoothly increase and not jump around at all. You can also test resistance of the TPS - I can't remember the values, so you'll have to find those in a Haynes manual. Also test the wiring between the TPS and the computer for continuity, and also check the signal and power wires for shorts to ground or power.

 

You have the EXACT same problem I've been having with my 90 for a few weeks. I've also done all the same things you've done (and more) with no permanent fix. I also gave up and took it to a dealer who couldn't find a problem so they told me I needed an engine! (Compression test was NOT that bad at 135 low and 165 high, they just needed a reason to get it out the door). I was able to make it better for a few days by replacing what appeared to be a faulty rotor, but soon the problems returned. I spent all day yesterday with a buddy of mine who is an ASE certified master tech and neither of us could find a problem. I've tried EVERY suggestion I've read on this forum without any improvement, and I got all the same ones you are getting. We tested everything under the hood no matter how simple or stupid. To top it off the truck ran worse when we were done!

I've given up a second time and taken it to a private shop a friend recommended as being real good. Hopefully they can find the problem and fix it. Unfortunatly they can't look at it for a week or two (which I guess is a good sign they are so busy?!) If they find it I will be sure to post back. The symptoms are too darn close to be coincidence.

By the way, mine is a 90 302 5-speed. I'm already $600 into this problem and getting REAL irritated. If they don't fix it I fear the Bronco will have to go before I'm broke!

-Jimmy

 

if I may say this I had a problem with my 95 F-150 that no shop had been able to fix. It hunted and seeked for idle at times. The engine would act like it was going to stall. Then would rev up to about 1000 rpm then calm back down. I came to find out that the problem was caused by the coolant sensor it was sending a signal to the comp saying that the engine was cold when indeed the engine was warm so it would force the idle up and deliver more fuel to the engine causing my engine to run rich and at a higher rpm. I fought this problem for 12 months three dealers, four private shops. Technology is great but it is as only as good as those that are paid to service it and have the knowlegde to do so. and no one time did my truck throw out a code.

 

Just a suggestion, but i would try a new computer or get one from a junk yard
when I work as a mechanic and we had alot of problems with fuel injected vehicles of all kinds we would plug in a new computer, if that didnt help it at least ruled that out.
But lots of times with gm passenger cars this cured the problem.