Ok my '86 F150 started acting up the other day. At anything above 1/3 throttle it loses power, bucks, and will backfire through the intake. Floor it in Neutral and it stumbles and pukes fire out the throttle body. I've pulled codes and I get a EVP sensor too low (I believe that the EGR solenoid is the fault there) and a MAP voltage low code. I've tested the MAP sensor and it tests within spec so I wonder if it's not a result of the backfire causing a MAP error - the code will go away for a while if I reset the ECU. I get good vacuum (18-19.5" at idle - throttle snap goes up to 20-21"), idle is rock steady, engine temp and oil pressure are normal.

Could it still be my MAP sensor even if it passes the test? I was thinking fuel pump but it acts the same on both tanks, and since it pukes fire out the intake I'd say it's getting enough fuel. Timing is good (10 deg spout disc. and advances with spout in), plugs, wires, cap, rotor are all brand new and all wires are firing consistantly so I don't think it's an ignition problem.

Any insight would be appreciated!

 

bwiecek
I would check the TFI module, it is the gray module that is attached to the outside of your distributor. If they are too close to the intake manifold they can fail because of heat.

 

Any electrical item can work fine at low rpm but fail at higher rpms.
Several things could be the source of your problems. I am going to list as many possibilities as I can.

1) A common symptom of clogged egr ports/valve/system is a pronouced miss at around 1500-2000 rpm, though it should smooth out at higher rpms
2) you could have a charging problem - ie your alternator (or the voltage regulator inside it) could be starting to fail. It may not be producing enough voltage at higher rpms.
3) Your Coil in the distributor is failing - this often show itself at higher rpms - but will run fine at idle
4) Never rule out bad gas - I don't think this would cause you codes but may cause the backfiring at higher rpms.

recommendation - check the cheap stuff first.
I would probably go ahead and change your fuel filter (just in case you are not getting enough fuel)
From there I would probably check and/or replace the coil in the distributor.
then move on to the most complicated things.

no guarantees, but hopefully this will point you in the right direction.

 

It sounds maybe like EGR. I would give it a few taps and see if it is stuck. Worked for my dad once when his ran like crap. Tapped on the EGR and the valve became unstuck.


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1998 Ford F150 4.2L V6
American Racing Rims
Brush Gaurd
Tinted Windows
Saving for lift and SuperChip
3.55 Rear
5 Spd.

 

what condition are your spark plugs in? carbon build up? How did u test your map sensor? did u test it through the entire range using a vac pump and monitoring the hz?

 

I really think you have a fuel pressure problem. EGR will not cause backfires, even if it is stuck wide open. Modules usually just quit and stall the truck. It REALLY REALLY sounds like fuel pressure--- your truck has three pyumps.. two lift pumps and the high pressure pump (for two tanks that is) Any of the pumps failing can cause low pressure as the high pressure pump cannot generate high enough pressure without the lift pumps. Beg borrow or steal a guage.

 

Thanks for the replies - here's the additional info...

1) Spark pulse looks good using a timing light and replacating the problem - the pulses are regular and don't skip/miss. it doesn't drop out at throttle/under load so I don't think it's a spark issue - I'm guessing that rules out TFI, Pickup coil in dizzy, HV coil, etc. I suspected TFI since it's a common failure point but it also occurs when stone cold and most TFI modules get worse as the engine warms - that's also leading me to think it's not TFI. I will test the resistance of the inductive pickup in the distributor tonight to be sure as well as the primary and secondary HV coil resistance just to be sure.


2) EGR valve is closed - it looks real new since I've had the code for the 4 years that I've owned the truck and the P.O. replaced the EGR and EVP trying to clear the code. The body is COOL to the touch after a 45 minute drive home. If I apply vacuum to the valve it opens and will stall the engine if I don't blip the throttle, once open it gets HOT real quick so I believe that the valve itself works fine just the vacuum solenoid isn't opening. I have 12V and 0V on the 2 leads of the solenoid at idle so I think it's shot.

3) battery is about 8 months old, running voltage at 13.8 idle, 14.2 @1500 RPM, cranking voltage 9.5v using fluke DVM.

4) fuel filter is about 2.5 years old.

I don't have a way to test fuel pressure so I don't know what it's doing at idle and under load.

Thanks for all the replies so far - it's a couple of things to check!

 

Well, the test I did was the one in the chilton's manual - measuring voltage at certain vacuum. I know it's not a 'perfect' test but I think it did an ok job. Plus if I clear the code it goes away temporarily.

Hmmm.... 3 fuel pumps!! OUCH - that's got to be $$ to fix - do you think it would cause backfire through the intake if the pump couldn't supply enough fuel? I have an autometer guage laying around from a mustang, just need to get the adapter to hook it up - I think it has a 1/4" flare fitting on the line to the guage and then from there to the rail does anyone know what the fitting is???

Fuel pressure sounds good - I assume that we're talking the high pressure pump here since it acts the same on both tanks right?

 

I think nomopar4me is on the right track. Leaving out major mech. probs. like jumped timing chain, which would run poor at idle too, backfiring through the intake or carb I've found to be a too lean condition. The fuel rail test ports are shraeder valves, just like suction side R-12 freon fittings. Before I bought a gauge with a purge valve, I used to put an old R-12 can-tapper hose on the rail to burp it into a can before undoing stuff in the fuel system. A key on-engine-off pressure check, then a running pressure check, is the place to start.

[This message has been edited by Granpa (edited 03-01-2001).]

 

Now that I've been noticing things I believe that the fuel pump sound is louder than I remember it being... I'm going to take a chance and replace it - a guage kit costs $70 and the pump is $74 so I'll just do the pump since it's noisier than I remember...

FWIW I rebuilt the engine about a year/5K miles ago (this was primarily used to tow a circle track car and doesn't see much use, but now I'm driving it while I'm hot-rodding my other truck)... everything mechanical is NEW in the engine and all checks out compression, leakdown, cam timing is tight, oil pressure is excellent, etc.

 

BW, be sure that the rear pumps are pumping. I have seen several trucks with BOTH inop. Pull the line off the BACK of the high pressure pump then cycle the key on. This should produce aspurt of fuel from the tank that the switch is on. Switch the selector, cycle the key off then back on to test the other tank. Unfortunately the High pressure pump can get noisy if it is not being fed adequately.

 

I agree with the others. Sounds like a fuel starvation problem.

Start with the cheap fix. Change the fuel filter. If that doesn't fix it, check the fuel pressure.

 

Check your Sears store for a fuel pressure gauge. Good quality, 30-40 bucks. Probably the most popular gauge around. Works on any fuel injected car or truck, even comes with instructions folded up in the back of the package. Should last a lifetime if not dropped on concrete (grabbed mine just in time once!).

 

Well, I purchased a decent guage for $36 from AutoZone - looks exactly like the sears one except the fittings are brass instead of chromed...

It confirmed the problem - pressure was dropping like a ROCK when the throttle was applied - at idle pressure was good ~28 psi and vacuum off the regulator it was ~44 psi but as soon as the throttle was pressed the pressure dropped! I can't believe the thing ran as good as it did with such little pressure! Hopefully it'll help my gas mileage - I've been getting ~10.5 mpg lately!

I've got a replacement frame-mount high pressure pump and a filter that I'm installing this weekend and should know if it's fixed by monday. I figured it had to be that one since both tanks are just low pressure lift pumps.

The only nagging question is that the pump provided enough pressure at idle it just couldn't handle the volume - so would that be the in-tank pump or the high pressure pump??? Any opinions on this one?

 

Thinking logically, if a pump can raise static pressure to spec, but can't deliver the proper volume, then it's probably something other than the pump itself. I'd look for a restriction somewhere.

It sounds like you have dual tanks and it happens on both? If that's the case, then you should be looking at everything from the transfer valve forward to the engine.

I'd do the easy things first like pinching every square inch of flexible lines between the fuel pump and the transfer valve. A bad hose could be collapsing. Are you sure the filter isn't clogged?