occasional misfire - 94 B4000

[curtis73]

Getting a frustrating misfire every once in a while. List of all the symptoms (maybe related, maybe not)

- sometimes when I start it up, I get a misfire; a little shake. It seems rhythmic like it's one cylinder, not random which makes me suspect ignition
- Yesterday after having sat for a week, it cranked a long time before starting which makes me suspect fuel pump
- Today I started it (hot) and it shook and shuddered at about 500 rpms before dying, then restarting was just fine which makes me suspect IAC
- doesn't seem to matter hot or cold and the randomness of it makes me NOT think vacuum leak, plus I just did an intake manifold gasket.
- No CEL/SES, but it is OBD1

90k miles. Plugs look fantastic, but looks aren't everything. Previous owner did coil and wires, but that doesn't mean they're good.

Where would you look first?

 

[RonD]

So it only happens on startup, no problems while driving?

I would cycle key on and off 3 times then try to start it, that will restore the 30psi fuel pressure that may be lost when key is off

Fuel pump only runs once for 2 seconds with key on, then doesn't come on full time until RPMs are above 400, engine started

Cycling the key 3 times will run the pump for 6 seconds, which gets 30psi pressure, which is what it should be, 30-40psi

There are 3 ways to lose fuel pressure with key off, not counting a broken fuel line, which you would smell
Check valve inside fuel pump, the backflow preventer valve
FPR(fuel pressure regulator) valve, is not closing tight
Leaking fuel injector

Leaking injector would cause Rich running and a big drop in MPG, so pretty obvious if thats happening

FPR and check valve leak pressure back to gas tank so no noticeable symptom except hard to start and stumbling start until pressure builds up
You can get a pressure gauge and hook it up to Schrader valve on the engine to see if pressure is the issue

 

[curtis73]

Ok, thanks. Your idea of injector could explain all of the above. I have a fuel pressure gauge so I'll check it. I guess if I hook it up and cycle the key and get 30-40, then let it sit, there would be a noticeable drop in pressure after 15 min or so if it were FPR or injector.

I have a set of gently used spark plugs. I may toss them in and see if it changes anything.

If it is running rich, it isn't enough to cause poor MPG or black smoke. Still getting 17-18 mpg.

 

[RonD]

Not an injector then, MPG would be under 10MPG

Yes, hook up the pressure gauge, see what it says
Turn key on one time, then see what it says and see how long it stays there, from 0psi it should go up to 10-12psi with one key ON
Then 18-20psi with 2nd key on
Then 26-30psi with3rd key on

With engine running it should be 32-38psi, fuel pump on full time
Turn engine off, pressure should hold above 15psi, 20psi is normal, so turning on the key one time makes it 30psi

There is a Vacuum hose on the FPR, unplug it with engine running and pressure should go up to 41-43psi

You can remove the Return fuel hose from the FPR, put a towel down under FPR port
Turn key on one time, look to see if fuel is coming out of FPR, it shouldn't, it should hold fuel until it gets to 43psi, engine off