# backfiring 1988 nissan pickup



## raymilosh (Jan 12, 2005)

Hi all,
I am baffled by this truck. Its a d21 with a 2.4l z24 motor with electro ignition. (Basically throttle body fuel injection.
It idles fine. It backfires and coughs and spits terribly under acceleration. when the gas pedal is released, it returns to a normal idle. 
It has 2 coils. We found that one misfired perfectly in time with the symptoms. We replaced it. Now it no longer misfires, but the symptoms persist. 
We have changed the distributor and the computer with no change in symptoms. 
The compression is good, the valves have been adjusted, the timing is a tiny bit retarded (6 deg BTDC instead of 10 deg BTDC), but can be advanced no further b/c the distributor cannot be turned any further. The timing advance is working. 

I had an F350 with the same symptoms...turned out the valve guides would get hot and grab the valves momentarily. We had the head reworked and the symptoms went away.

So, anybody got any ideas why this engine won't accelerate?


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## agmantoo (May 23, 2003)

Disable the egr valve temporarily and observe the results


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## wy_white_wolf (Oct 14, 2004)

SWAG:

Timing belt has jumped a tooth (or was installed a tooth off) and to try and compensate someone adjusted the timing. That's whty you have no more adjustment with the distributor.

WWW


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## raymilosh (Jan 12, 2005)

Holy cow, www, that might be it! Brilliant idea. I cant wait to check it out tonight.

Agman...how does one disable the EGR?


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## tinknal (May 21, 2004)

If you are going to check out the timing belt, replace it while you are at it. If an old belt breaks it can destroy some of those Japanese engines..


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## agmantoo (May 23, 2003)

raymilosh 

one of two methods are used depending on the vehicle
older vehicles tend to be vacuum controlled. on those remove the vacuum line and then plug the line with a pencil or a small shaft that will slip fit into the hose.

on the other method it is electrically controlled by the computer. just unplug the wiring that does to the EGR


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## OkieDavid (Jan 15, 2007)

Had an 86 Nissan w/z24 engine that did that when the O2 sensor went bad. Symptoms were not like ANYTHING I expected with an O2 sensor but the mechanic got it right on the first try.


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## raymilosh (Jan 12, 2005)

Rats, it has a timing chain and it looks fine. 
The EGR is all solid pipes with one vacuum solenoid. I unplugged it. No change. 
The symptoms are different than I originally said..
The engine runs and accelerates fine for about 1 minute with no problems at all. The first symptom is sputtering under acceleration. It isn't backfiring through the carb. If I floor it, the engine acts exactly as if I have turned off the ignition. If I keep it floored, it won't fire again and will coast to a stop. if i back my foot off the pedal at any time before the engine stops, it fires and runs again. I checked the sparks coming from both coils and they keep firing the whole time during all symptoms. It simply must be the fuel supply being absolutely cut off while the pedal is depressed. If I keep trying to make the truck keep going, the problem gets quickly progressively worse to the point that it will only continue running if I get my foot completely off the pedal. If I let it rest at idle a moment, the symptoms back off a bit to where I can accelerate a little again before it cuts off again. 
My fuel pressure gauge maxed out at 10 psi when we teed it into the intake side of the Throttle body. The fuel pump does seem to be delivering alot of air with the fuel. Bleeding off the air does seem to make it a little longer before the symptoms return.

I'm a little outta my league here...something seems to cut of the fuel supply to the injector at full throttle, then mid throttle, then anything off of idle speed. Is this to do with a throttle position sensor? I understand the crank position sensor is in the distributor, which has been replaced. 
hmm. mystery


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## Travis in Louisiana (May 14, 2002)

Could the suction side of the fuel pump, or line to it, have a hole in it to where when you need more fuel, air is sucked in with the fuel to the throttle body, thus causing the air in fuel problem? How full is the tank?


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## oneokie (Aug 14, 2009)

Have you replaced the fuel filter? Does it have the external electric fuel pump? If it does, replace the flexible line from the tank to the pump. Also check the lines from the charcoal cannister back to the tank. A SWAG is that it could be that the tank is not venting properly and a vacuum is forming when fuel demand is greater than the amount of air being vented into the tank.


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## agmantoo (May 23, 2003)

It is a fuel related issue with that description of the behavior. It is starving for fuel when under load.


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## clovis (May 13, 2002)

I agree with agmantoo. Sounds like it is starved for gas.


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## raymilosh (Jan 12, 2005)

huh, O2 sensor,eh? I'll check it out.
The fuel pump is inside the tank. We removed the fuel line from the carb and caught the pressurized fuel in a bottle we emptied specifically for the purpose. we noticed that a lot of air came out with the fuel. we tried this several times and each time had a lot of air come out. we wondered if there was air being drawn in before the fuel pump. we put in more fuel..now it has about 6 gallons in it. the problems persisted. i convinced myself that the air is not a problem b/c if it was airlocked, why would it resume idling normally when it is allowed to return to idle. Is my logic faulty?
I ran a fuel pressure gauge in the cab. the pressure stays at 42 psi. sometimes it drops to 36 while the symptoms happen, sometimes not. The fuel pressure is supposed to be at 36. I need to see if the fuel pressure regulator is working, but i don't think that's the problem, either. 

Ill go check the computer codes to see if it has thrown anything and start just throwing parts at it after checking out the function of the O2 sensor.

In the meantime, I'll spend some changing the fork oil on the motorcycle...i need to regain my confidence that I can actually accomplish something. shesh.


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## agmantoo (May 23, 2003)

raymilosh

This is about as far fetched as I can go. Most in tank fuel pumps are IMO gear drive design. There is a fine mesh filter on the inlet side of the pump. These filters will/do clog. If that filter clogs and cannot allow enough gas to enter the pump the pump could cavitate and could create air in the fuel line. The fuel pressure could still read OK but you will sporadically get air instead of gas into the combustion chamber creating an erratic firing. Regardless, air in the fuel line is not correct!


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## bikehealer1 (Oct 8, 2009)

its probably the line connecting the fuel pump to the rest of the sending unit. when they get old the lines can split and allow air to enter the circuit. had this happen on a 93 S10 blazer.


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## agmantoo (May 23, 2003)

bikehealer1
With the fuel pump in the tank then any line outside the tank would be pressurized and then would be leaking fuel would it not?


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## SteveO (Apr 14, 2009)

How about something way out there. a broken motor mount when you put it on load it twists and pinches the fuel line??
Steve


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## Guest (May 1, 2012)

I would change the fuel filter . I had a truck that would idle fine but when the throttle was pressed the engine would die . A new fuel filter fixed the problem . Might not be your problem but would be easy & inexpensive to try .


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## bikehealer1 (Oct 8, 2009)

agmantoo said:


> bikehealer1
> With the fuel pump in the tank then any line outside the tank would be pressurized and then would be leaking fuel would it not?


the line from the pump to the sending unit is inside the tank.


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## dirtman (Sep 15, 2011)

I don't know if they had catalytic converters in 88 but when the one on my dodge truck went bad I had pretty much the same conditions you have described.


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## Darstcreek (Apr 28, 2012)

Clean or replace your throttle body then use Sea foam in 2 straight fill ups !


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## raymilosh (Jan 12, 2005)

update:
I unbolted the catalytic converter. More power, lots louder, but the symptoms persisted.
I filled the tank with fuel (thinking that if the fuel line was sucking air, it would be covered with fuel and would now suck in fuel, rather than air.) The symptoms didn't show up until i was driving for 10 minutes and were very much reduced, but still present...sort of. I guess i gotta empty the tank now. rats.
and thanks for your typical keen pointing out of things that ain't right, agman.


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## DryHeat (Nov 11, 2010)

Just noticed this thread. What you report does remind me of problems I had many years ago when I filled my van with apparently "bad" gas, contaminated with debris, launching out from Denver to back east one summer. An inline filter clogged up and wouldn't allow adequate flow at higher throttle after I got maybe 50 miles out onto those endless prairie distances. May have had some symptom changes as the gunk shifted around in the tank, and some improvement with dilution when fresh fuel added after draining the tank. Filter replacements on road let me limp home but as I recall tank had to be completely cleaned. This was model before computer controls, and didn't have in-tank pump, though, so may only be apparent similarities. The other suggestions certainly sound plausible, too. Good luck...


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## raymilosh (Jan 12, 2005)

i pulled the tank out and it was full of rust, so I cleaned it. I haven't reinstalled it yet. There is no fuel line between the screen and the fuel pump, the screen is attached straight to the pump. There is, however, a small diameter hollow plastic pipe factory fitted onto the fuel screen that extends upward, ending near the top of the tank and the end is left open. What the heck is that for? Could it have been sucking air?


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## agmantoo (May 23, 2003)

raymilosh

Is there any fuel line other than metal going from the pump to the outline/connection ?
Is the screen on the pump inlet side clogged/restricted? If the inlet to the pump is restricted I believe the pump could cavitate. Highly refined fluids, such as gasoline blends, can cavitate.


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## raymilosh (Jan 12, 2005)

The screen was the problem. I put on a new screen that does not have the air pipe leading out of it, reinstalled it and the truck now runs fine. Whsew.


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## agmantoo (May 23, 2003)

raymilosh
Congratulations!
As I have stated before....Anything can be made to work if you work on it long enough! : )


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## OkieDavid (Jan 15, 2007)

Good job! Nice to see you work through the process and solve it.


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