87 Bronco II severe missing & sputtering

[CraigRK]

1987 Bronco II with 2.9L V6. Engine starts and runs fine for a short time. Within about 5 minutes, the engine begins to demonstrate cycles of severe missing(?) and sputtering(?). These go in regular cycles, with the motor acceleraating fine, then when load on the motor decreases, it suddenly starts to cut out and sputter, then when the rpm's drop to about 1800 or below and there is once again load on the motor, it seems to catch (almost as sudden as if it was on a switch) and begins to accelerate again, only to die/sputter again once the rpms reach about 3000 and/or the load is decreased. Along a straight section of road, these cycles occur about once or twice every minute. Gas mileage is VERY bad, and the vehicle will not pass the MD emissions test.

I have replaced the plugs, wires, cap & rotor then the entire distributor, TFI unit, & MAP sensor with no improvement. Changed the fuel filter twice (no difference). Using an old tire gauge, it looks like fuel pressure in the manifold is A-okay. The exhaust system has been rebuilt from the exhaust manifod back. The copper pipe connecting the manifold air reaction/injection(?) unit to the exhaust manifold port has been removed, and has both ends plugged, but FORD dealer mechanics said this would NOT affect the engine. The vehicle is still in good condition, but is almost unusable with this problem. My teenage daughter is about to get her drivers licence and wants to use this vehicle. Any insight into this problem would be appreciated. Thank you.

[soulsurvives]

i think you are having the same problem that a friend of mine is having.....we have done all that you have and got no results....today after work im replacing the coolant temp sensor...this tells the computer how hot or cold the engine is.....when its cold the engine needs more fuel than when its warm.....i think thats why it runs good for a few minutes then starts runnin like crap.....i will let you know if this works.....

[CraigRK]

soulsurvives

Thanks for your response. If we have the same problem, I doubt the coolant temp sensor is the problem. My temp sensor was working fine when this problem developed, and then I accidently broke it off (completely non-functional). No difference. Also, the severity of the problem seems to be much more than just an improper fuel mix adjustment for temperature. When the missing/sputtering kicks in, it is VERY bad. (Actually, its gotten worse over time - how's that for good news!) And when the missing problem stops (each cycle) it is very sudden and the engine kicks in and runs fine. (For a half a minute.) I cannot determine if it is an electrical issue (it's just like it suddenly runs on half the cylinders, then kicks in on all 6 again), or a fuel supply or fuel mix issue.

My problem does seem to be related to load on the motor (possibly vacuum related???). When I accelerate, making the motor work, the problem pretty much goes away. It's only when it hits a cruising/low load condition that the sputtering/missing cycles start. Have you tried changing the coil? (I haven't yet.)

Please keep me informed, and I'll do the same.

[Austin8214]

Sounds like an O2 sensor to me. Is your Check engin light ON? Have you had the computer scanned for codes. I think auto zone will scan for free but be carefull what you buy from them. They may try to sell you a bunch of non-returnable parts that will not fix the problem.

[CraigRK]

Austin

The Check Engine Light is not on on my vehicle, and hasn't been thru the development of this problem. Would this be on if there was an O2 sensor problem? Would a computer scan yield any useful information if there is no Check Engine signal? (BTW, thanks for the Auto Zone tip. I have installed, tested and then returned several items, such as TFI and MAP units.)

[soulsurvives]

well here is a progress report.....temp sensor did no good.....but it was the cheapest start....today im gonna put on a new fuel pressure regulater and an idle air bypass valve.....when this thing is not trying to stall it idles very high......also do these things have an egr valve.....as far as i know the check engine light has never come on on this truck either.....what about the possibility of the computer being bad.....also when yours starts to spit and sputter do you get a shytload of black smoke commin out the tailpipe.....this one does...then it will stall.....kinda like its dumping to much fuel for the motor to burn....wonder if the ground for the coil could be crappy.....anyway i will be trying some stuff today after work.......i will keep ya posted....

[CraigRK]

soulsurvives

Thanks for your report. Too bad the temp sensor didn't solve the problem.

Yes, I get a lot of black smoke. I have a big black exhaust patch on my white garage door where I ran the vehicle on the weekend. And the gas mileage is very BAD, as if it is running way too rich, and the gas mileage is much lower than a lean problem or electrical miss might cause (~8mpg!).

Here's another reason why I think it is a fuel vs. electrically related problem. When I hooked up a timing light, the strobe light does not cut out when the sputtering begins. Also, the tach doesn't show a drop any larger than that due to the reduced rpm's. So if the problem is electrical in nature, it's likely not a complete not-firing, but some sort of reduced firing issue. (Would a weak coil do this?)

I'm pretty sure these motors all have EGR valves, if you give me your motor year and model, I'll look your type up in my Haynes manual (not the greatest manual BTW). Mine is the ported EGR type, without the electronic EVP on top.

Question: Is your gas recirculation pipe and system still intact? Including the 1/2" copper pipe that runs from the intake manifold to the exhaust pipe?

[orion96]

Hey buddy,
I'm kind of an amature at this, but r u sure that when you replaced the wires you checked to make sure that you put the plugs back on the distributor in the correct firing order?

Good luck

[Truck]

You might also check the wiring harness for the fuel pump. When they get old and corroded, they might miss-as well.

[Austin8214]

Craig,

A scan may reveal what they call a pending code. A pending code does not turn on the Check engine light. Also with a scanner in the hands of the right person they can look at the readings being put out by the different sensors.

[soulsurvives]

well i think the problem has been solved.....last night i replaced the iac vavle....but i think that wasnt the problem anyway.....i put a new pvc valve in and i adjusted the idle......i also found a vacumn line that was not hooked up......also the evap canister was laying on its side so i set that up right....well last night it ran great and today she told me it ran great so im keeping my fingers crossed.......we are gonna put on a new high pressure fuel pump on cause the one on there is makin lots of noise.....but she is runnin good so far........i will keep you posted....

[CraigRK]

soulsurvives

Glad to hear your problem is solved. I think I am on a convergent path. I noticed yesterday that the rubber grommet in the valve cover that holds the PCV valve was worn, and the PCV valve was sitting crooked in the grommet, possibly leaking vaccuum. This morning I took out the PCV valve and plugged the valve hole and the vaccuum hose with plastic plugs. The vehicle runs MUCH better. I still get a little bit of the missing/sputtering cycling, but it is much reduced, at a higher rpm, and shorter duration. Almost livable. I am starting to think the problem is multiple leaks in the vaccuum system, and I'll have to chase these down one by one.

What is the iac valve? I have an "Air Bypass Valve Assembly" on the side of the upper manifold, could this be it?

Austin - thanks for the info on the electronic scanners.

[Austin8214]

You can use WD-40 or something like that to spray hoses and look for the leaks. When the leak sucks in the WD-40 you will notice a differance in the way the engine runs it should smooth it right out.

[soulsurvives]

craig.....yes the iac is the valve on the side of the the intake.....but i dont think that was bad tho....yeah little vacumn leaks can play havoc on an engine.....well hope you get it all worked out....so far my friends is runnin good still......

[acidburn5377]

give the egr valve a shot and the pipe connecting it to the exaust manifold my 87 b2 had the same problem and replaceing the egr and that pipe fixed it. its worth a shot