Missfire on startup, low fuel pressure

[JustinY]

I have a 92 explorer, 5 speed, that will randomly have varing degrees of missfire on startup. 50% of the time its missing on maybe 1 or 2 cylinders, but sometimes it wont run unless I stay on the throttle and is only going on maybe 2 or 3. After driving for about 30sec-1min, it runs fine. I did notice an odd gurgling sound coming from the fuel tank when it's at it's worst. I checked the fuel pressure while it was running fine and according to the chilton manual it was 3psi low of the range with and without the vacuum line on the regulator. I also checked the plugs and they all looked fine.

Is the slight low fuel pressure the cause? Considering there is a pressure range, and it is below that, I figure even 3 psi would cause issues. If so, what could be causing it? I assumed fuel pumps just went out, not lost pressure. I also dont know when the filter was last changed.

The problem is slowly getting worse over time, but seems to be the very worst after sitting hot for 15-30 min or so. Also the hotter the weather, the more often it happens too.

Thanks for any help.

 

[arco777]

Check fuel filter, fuel pump, pump relay. They do get weak. Check that the regulator holds pressure. Also check the spark plugs for tiny cracks in the porcelain insulators, as those can cause misfires until they heat up.

 

[Maniak]

There is supposed to be a check valve on the Fuel pump assembly. It is really just a rubber hose with the end kinda squished. If that goes bad you can loose fuel pressure when the vehicle is off..

When that happens it may take a few cycles of the key (turn to on, off, on,off,on,off,on, start) to build the fuel pressure. Once you have pressure it should fire right up..

This is assuming you don't have a bad FPR, plugged filter or weak pump like arco777 mentioned above.

~Mark

 

[acschilling]

My 92 5speed will do the EXACT same thing! Finally this shows up! I think the fuel is bubbling in the rail..New pump, fpr, filter, and evap canister (which did help my fuel economy). I cut THE HOLE under my seat last time I did the pump and didn't notice anything pinched.

I just say the fuel is bubbling because if I park it on the black gravel part of my driveway when its hot the misfire is baaddd..

Ends up driving just fine tho.

 

[arco777]

Might want to double check the plugs. I just changed mine today (way ahead of schedule) and of the Motorcraft Platinums I had installed 10,000 miles ago, three of them had cracked porcelain insulators. Replacing them fixed my cold start misfire.

Can you isolate which cylinders are misfiring? That will often make it easier to track down the cause. Use an infrared temp gun if you have one to check outlet temps at each exhaust manifold point (look for a cool one), or pull spark plug wires off (welding gloves work well) at the coil and see which one(s) have no effect on the idle quality. When I was troubleshooting my misfire both methods pointed to the cylinders with cracked spark plug insulators.

 

[2stroke]

Wow, I have been searching for issues with my EX for a few years, and now that I'm no longer willing to fix it people come on and describe the EXACT same problem? This is the 2nd time this week. I went through everything, never got a code for it, new fuel injectors, new fuel filter. I've come to the conclusion that it is the fuel pump, and exactly like you say, it only does it when it gets hot. One other thing is to replace the relay for the fuel pump. I was willing to fix it a while back, but everyone kept telling me that fuel pumps are either good, or bad. Now I know that they do cause intermittent problems, but I just live with them. For what its worth, anytime it was so bad I couldn't drive, I simply swapped the fuel pump relay with another relay, and for whatever reason it fixed it until the next time the fuel pump got hot. Then you can swap them back, and it runs again.

 

[Maniak]

My 92 5speed will do the EXACT same thing! Finally this shows up! I think the fuel is bubbling in the rail..New pump, fpr, filter, and evap canister (which did help my fuel economy). I cut THE HOLE under my seat last time I did the pump and didn't notice anything pinched.

I just say the fuel is bubbling because if I park it on the black gravel part of my driveway when its hot the misfire is baaddd..

Ends up driving just fine tho.

If the fuel is bubbling in the fuel rail, then you don't have pressure which means you need to fix the loss of pressure. When the fuel is under pressure it won't boil but if you have leaking injectors, or check valve or some other leak you will lose pressure and then it is possible for the fuel to boil BUT its pretty unlikely.

Even if it did, once you get the pump running the pressure will build and it would stop.

~Mark