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Discussion Starter · #1 ·
I have an 1986 5.0. Starts fine, idles fine (1000 RPM), accelerates fine until a magical 3400 RPM, the engine starts to cough and hack. It will back fire if I keep the throttle there.

The vehicle sat for 6-8 months while I put in new gas tank, skid plate, fuel pump, and I ran large diameter stainless steel braided line from the tank to the fuel rail. I could have some junk in the injectors.......

Have searched and have the following data.

Base Timing 10 BTDC (SPOUT disconnected)
Timing w/SPOUT about 20 at IDLE.
Timing as I accelerate almost 30, backs off a bit as I continue to open TB
Fuel Rail Pressure 40-37 PSI (as I accelerate)
KOEO A couple of code - random stuff
KOER test - no codes
Center of rotor: A little more black than I would like
Edge of rotor: A little scaring and black.
I did notice that the timing light seemed to go a little inconsistent as I hand operated the throttle.

I will replace them Monday night but....

Could a cap and rotor cause this sort of behavior?

Any thoughts?

SJ
 

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Lick my balls
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When is the last time you did a complete tune up?
EG: new cap, rotor, plugs AND wires?
Is it time for a new timing chain?
 

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KOEO A couple of code - random stuff
KOER test - no codes
What are the exact codes your getting for both of these?

Once we get these error codes then we can resolve those which might be causing your issues.

Did the KOER give you a code 11 or nothing at all? If nothing at all your EEC may be bad, you should get a 11 to say it passed the running test.

Backfiring can be caused by bad plugs, wires, cap, rotor or one of the KOEO error codes.
 

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Lick my balls
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I will run the test again to see the codes. I did get 11.

Cap - Rotor: Will do Monday.
Wires and Plugs: Done by Dealer in 2006.
and how many miles have you put on it since then? What do the plugs look like?
 

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Discussion Starter · #6 ·
Miles: A couple of hot desert trips per year. This is basically a trailer queen. I've spent a lot of time restoring it. So sitting in the garage most of the time.

Wires look brand new. I haven't pulled the plugs yet.
 

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Kitteh Commandaar!
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When you open it up, does the exhaust seem to get a little black, like it is running to rich? Could possibly be a coil thats on its way out, possibly TFI (assuming your truck is EFI, but I doubt it's the TFI) Check out the rest of the ignition components. Maybe even check the voltage TO the coil to see if it drops as 3400 rpm


KC
 

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I have an 1986 5.0. Starts fine, idles fine (1000 RPM), accelerates fine until a magical 3400 RPM, the engine starts to cough and hack. It will back fire if I keep the throttle there.

Any thoughts?

SJ
You know, I had an 86 years ago and it did the same thing. After weeks of it doing the backfiring, someone called the cops on me and said I was doing a drive by and shooting out the window. Got pulled over and searched and they found nothing. Your fuel filter is most likely to blame. Its in a canister on the frame rail and an oil filter wrench is needed to unscrew the canister open. Be cafeful as I've heard they break easily. Mine didn't though. I had to buy a filter wrench that was adjustable to get it to come off. This filter wrench had like a rubber pad made into it. Hope this helps.
 

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Satyr of the Midwest
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If by "inconsistent" you meant that the timing mark appeared to be moving back and forth, then that's likely a large part of the problem. That could be caused by a worn timing set, worn distributor/cam gear(s), a bad PIP, bad harness connectors/wiring, or a bad ignition module. A malfunctioning ignition could be causing your engine to dump raw a/f into the exhaust stream, and it's igniting whenever a hot exhaust pulse hits it. A lean misfire condition can also cause a backfire, but a faulty ignition system should be ruled out first IME.

I'd also look for exhaust leaks, bad check valves, or cracked manifolds.
 

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Discussion Starter · #10 ·
When it starts to chug there is a brown plume that goes out.

I did cap and rotor this morning, no change.

I will do fuel filter, coil. The pressure stays pretty high, but it does move as the throttle opens (fuel regulator seems functional). What should the pressure stay at?
 

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Discussion Starter · #11 ·
I need to run the test again to be sure, but by inconsistent I mean that at the high RPM the light seems to "miss" like there is random missing spark (on #1) to trigger the light or the spark is coming too fast for the gun to keep up.
 

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Lick my balls
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I need to run the test again to be sure, but by inconsistent I mean that at the high RPM the light seems to "miss" like there is random missing spark (on #1) to trigger the light or the spark is coming too fast for the gun to keep up.
Then the problem is somewhere in your distributor.
 

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Not hard at all, just mark the direction the rotor is pointing and to what cylinder (assuming the cap is not wired to stock, like mine) and re and re.
 

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It's really easy, remove the 1/2" bolt and put it back in.

The key is to make a note where the rotor is pointing so you can put it back in the same location. Make a note where the #1 position is on the cap so you can get the dissy back in the right place. You may have to restab it a few times if it doesn't line up exactly in the correct location. You may need to slightly rotate the rotor a tooth to get it to slide in.
 

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Discussion Starter · #16 ·
Waltman: what does "(assuming the cap is not wired to stock, like mine)" mean?

Does anyone run the MSD / Mallory with magnetic triggers? Are they worth the money?

The instructions for them are very different. The MSD instructions are more what you have both described. How do you rotate the oil pump shaft it the disto won't fully seat?
 

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Lick my balls
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Waltman: what does "(assuming the cap is not wired to stock, like mine)" mean?

Does anyone run the MSD / Mallory with magnetic triggers? Are they worth the money?

The instructions for them are very different. The MSD instructions are more what you have both described. How do you rotate the oil pump shaft it the disto won't fully seat?
What I mean is the distributor is wired a certain way from the factory, but some people like myself wired it any way they feel like it. So if you were to look in the Haynes manual and tried to wire the plugs on my truck like it says, then you would never get my truck running.
 

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You don't you pickup the dissy a little and slightly rotate the rotor. it's not a straight down stab, the gear on the end of the dissy has to slide in the cam gear. When you stab it you will see the rotor rotate just a little and it will seat all the way down.

If not get the 10 lbs sledge hammer out and after 10 hits it will be seated properly.
 

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Discussion Starter · #20 ·
Found It!

I found the problem.

I kept digging into the spark issue. Was missing random spark on all cylinders. Checked coil input. Timing light was flahsing like mad but at the 3400 RPM point I could tell it was missing spark.

Pulled coil and put a junk yard coil in and had instant correction. Coil body was loose on the metal chassis.

Thanks for all the input!
 
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