Lack of Power and miss firing?

[Bansheemanche]

Ok I am running out of ideas with this truck. I have replaced my coil and ignition module to fix a severe missfire around 1800-2200 rpm. I have done a full tune up obviously before this. I replaced the coil and ignition module and it run better for a day or so then it started missing under light throttle for again, not as bad as before. Well I continue to drive it for awhile then one day it starts running like it is breathing through a straw. No power, I couldn't get it over 25 mph unless it was flat or going downhill, no acceleration going up a slight incline. 0-60 in about 5 minutes, no joke.

 

It cranks for a long time when I start it and i find if I cycle the key and prime the fuel pump alittle more then it fires right up. So I am thinking it might be more fuel related? Tried different gas, got a new gas cap, fuel filter and nothing worked. Sunday I drove it and it was dangerous to drive on the streets as I couldn't keep up with traffic anything more then a feather of the throttle pedal it would cut out completely, if I let up it would fire again.

 

So I parked it till tonight when I go to take it out to try it and it runs fine. It has power and but still has the slight missfire on light throttle. Under WOT it would run fine but I could feel a slight drag on it and it would fully kick in and then it would go back to feeling like I am towing something.

 

So I don't know what to do look for, fuel pump, tps or what? It isn't consistant at all, one day it has power and the next it feels like I'm driving a semi.

 

I know I need to test my fuel pressure and see but I did replace the stock injectors with used ford 19# injectors back in december. Ran fine before and after I did that.

 

It is on my 88 4.0L 5 speed Comanche.

 

Any Ideas, I really need some help here.

 

Thanks

[jtdesigns]

I'm having the same problem... >>HERE<<

Are you getting any backfiring through the intake at WOT?

[Wildman]

Really sounds like a fuel pump problem.........get the fuel pressure test, but that might not show the real problem.........like the pump is putting out 39 psi at the time of the test.....and as it get warmed up, running for some time, starts to fail (I had that problem on a Ford - let it sit for 15 minutes and it would run fine, after the pump 'cooled' off)

 

Another member here had a similar problem, and found the pump supply tube in the tank had split and was leaking inside the tank and not delivering the proper pressure/flow.

 

Might be worth the time to pull the in tank pump, and check out the supply tubes, the sock filter, or even change the pump.

 

Also, run the check on the ballast resistor, by jumping the wires and see if this is causing the fuel pump to not operate correctly.

 

Another item that will cause what your describing is the catalytic converter, a "plugged" cat will create so much 'back pressure' that it will rob the power right out of the engine, and this is a item that will run fine one time, and the next time it will feel like someone stuck a potato in your exhaust pipe.

You know.......fuel / air in.......and no place to go :doh:

 

You didn't post the mileage on the truck, but even with these "20 year old trucks" things start to fail :headpop:

 

Next thought........O2 sensor..... :hmm:

 

The CAM-shaft position sensor could also be causing a problem with the erratic ignition.

 

Your just going to have to spend some time on checking one thing at a time, and see if that clears up your problem.

[jtdesigns]

On my problem, I tested fuel pressure which was low, and didnt hold pressure. I then did a pump flow test and the result was well above specs. I bought new injectors (havent installed yet, but willl this weekend per 72* sunny skies) because I had fuel built up in the injectors connectors. Hopefully the injectors will bump my pressure up and it'll hold pressure. O-rings were good no leaks to other then the connectors filled with fuel. When I pulled the fuel rail and put some air into the regulator and it sealed pretty good, so I'm pretty sure thats not the problem.. Tested the pump resistor and it didnt change pressure.

The temp of the engine doesnt matter it does the same thing when just started or been idling till warm..

TPS is in specs. I still have to test that Cam sensor..

 

I've seen quite a few of these problems pop up here lately..

 

Hey Bansheemanche, here's a list of great links to help troubleshoot with;

>>Fuel Pressure Tips<<

>>Renix Sensor Breakdowns<<

>>Link to other tips<<

>>Specs on most sensors<<

 

 

I hope you , or I find the problem...

[Bansheemanche]

Thanks a bunch guys. I was able to borrow a fuel pressure tester and at first fire it was at 32 then dropped to like 28. I reved it a it jumped to 23. I shut it off and restarted it and it started at 26 then dropped to 23.

 

I did gut my cat alittle while back so that isn't an issue cause I thought it might be clogged also.

 

The truck only has 106k on it.

 

Thanks

[Wildman]

Yup......sounds like a fuel pump problem......

 

I should have posted this for you, there are 2 steps to follow on testing the fuel pressure, one with the pressure regulator connected, and the 2nd with it dis-connected.

 

http://www.lunghd.com/Tech_Articles/Fue ... ostics.htm

 

Also, it might be worth to pull the fuel pump first to see if the supply line is intact, and also check if the sock filter is plugged up.........your having kind of a range of pressures on your test.

 

Also, do the ballast resistor by-pass to see if that changes the range of your pressure tests.

[Bansheemanche]

Ok so I checked the pressure again, and I noticed that when I prime the pump there is no pressure, I only get pressure when it is running after it cranks awhile. Then I turn it off and the pressure reading on the gauge is nothing. So what does this tell me? Possibly the regulator is going out or is it still the pump?

 

Thanks.

[jtdesigns]

Pinch the return line when your trying to prime.. That'll tell you if its the regulator...

Its kinda hard to get to though.. follow the metal line going out of the regulator it will connect into a rubber line, take some pliers and slightly cut the return off for a few secs and see if you get pressure..

[Wildman]

:agree:

 

If you pinch off the return line, that would run the pressure right up, but only do that for a minute or less.

 

That would rule out the regulator.

 

If your not getting any pressure at the beginning of a start, that would rule out the ballast resistor. As the pump gets full voltage at the beginning of a start, and then routes thru the resistor on the run cycle.

 

It's got to be a problem with the pump, sock filter, or feed tube, pull the assembly out of the tank, and check it out, and next step is to replace the pump and filter.

 

IIRC most of the pumps were the Bosch pumps, Advance sells a complete kit for the replacement, with the filter, feed tube and O-ring, for like $90.

[Bansheemanche]

Ok so today I went to swap out fuel pumps. I get under there and see I have access to it without dropping the tank. So I take the old one out and swap out the new one. Now I have a hell of a time getting the sender to seat right up on the tank, finally get the ring on and go to try it out, here it prime then it just cranks no fire. I have no fuel pressure at all now. So I go and take the ring off again thinking it isn't seating right and do it again. Still nothing but the ring goes on but the sender doesn't look like it is perfectly centered, but the ring did lock it in.

 

So my question is, is it impossible to get the sender to seal right without the tank out and turned on its side so I can have gravity work with me instead of against me? Anything else I'm missing? The grommet that holds the bottom of the assembly was rotten and just fell apart so I left it off, that is just to hold it not have any fuel going through it does it?

 

Thanks

[Wildman]

I know the sender unit is a 'little' tricky to get in, but the only thing you really need to watch is that the pump and sock filter is sitting in the reservoir pan, that's in the tank.

 

Your not really fighting gravity on installing the sender unit, as it hangs down.

 

I would pop the supply line off at the tank, add a longer fuel line, and put it in a gas can, and hit the key for a second and see if the pump is suppling any gas. If it does, then you need to check the lines from the tank to the fuel rail.

 

If the lines were primed as you said, then you should have some fuel pressure.

 

Which grommet are you missing??? the one at the sock filter, or one of the plastic ones that support the pump its self??? If you don't have the pump supported, it can twist and turn inside the tank, and come off the supply line.

[Bansheemanche]

I will try the pulling off the feel line and see if it pumps out gas. I can hear the pump trying to prime and run when I hit the key.

 

It is the grommet next to the sock filter the fits on the sending unit assembly. It was definately rubber though. I don't know how I would replace it without getting a new tank though.

 

And I am right though, the tank really doesn't need to drop the tank as I got everything swapped over? Then if I test it and no fuel comes out of the supply line then it isn't sealed right it won't pump or is it the new pump?

 

Thanks

[Wildman]

I think the grommet your talking about, just fits on the bottom end of the sender unit, just to take the bounce out of the bottom of the tank. Unless you mean the grommet that holds the sock filter to the bottom of the pump, but it don't sound like it.

 

You wouldn't have to replace the tank, you would need to replace the sender unit, and they can not be found at any parts store.

 

Then if I test it and no fuel comes out of the supply line then it isn't sealed right it won't pump or is it the new pump?

 

Yes, either the feed line in not correct, the sock filter is plugged, or.......could the wires be on backwards, making the pump run backwards????

 

Yes, there is a chance the pump is faulty, it's happen before......was this a brand new pump from the parts store, or a used pump???

 

I've gotten a bad fuel pump for my cuber right from the Ford truck Dealer, and it was faulty from day one, and I didn't do any thing till 11 months later, when I broke down out of state, and then returned the bad pump, and got my $326 back. So much for buying parts from the dealer......in some cases. Been running a Napa fuel pump for the last 4 years now.