another MFI swap candidate?

[mixin]

Not to lead you to the answer, but the info I have might be pointing me there or to a cam position sensor

Anyways, truck has random misfire when cold under lite throttle. After 20 min or so, miss goes away. I have owned the truck for 2K miles and had miss when I got it, but not bad. Miss has gotten progressively worse with the same conditions as stated above. Finally set a p0300 code last nite when cruise was on and truck was going up a slight hill with the converter locked up and engine misfiring.

SO, after much research on automotiveforums, I borrowed my buddys Actron fuel press tester and got the following results:

Warm engine - 40min after a 30min drive

key on and engine off- jumps to 64 psi with pump on, as soon as pump turns off drops to 58psi, let sit for 1 min and psi creeps up to 64psi and stops.

engine running - 56psi, shut engine off- drops to 53psi and creeps back to 56psi after 1min

COLD engine - let sit for 4hrs

key on engine off- jumps to 64 psi with pump on, as soon as pump turns off drops to 61psi, let sit for 1.5 min and psi drops up to 46psi and levels off.

engine running - 56psi, shut engine off- drops to 54psi and creeps back to 60psi after 1min

Wanted to get psi reading while driving, but didnt think gauge would reach outside of the hood for me to drive truck and watch it!


Anyways, as for parts replaced, I have changed fuel pump assy. (with AZ brand tho-I know I know), plugs wires cap rotor (with prem AZ brand, bosch? i think) about 1.5 years ago when my buddy owned it.

other thread about cam pos sensor I read stated that in open loop engine relies on cps (if it is bad, eng runs bad). Once in closed loop, engine will not use cps and will run better. Is this right? If this is true it kinda fits for my prob, but I would think my PCM would go into closed loop after the thermostat cycles once. My truck misfires for 10 or more thermostat cycles before the miss goes away.

all right, let me know what you think and if you need more info I will do my best to get it tonite. I will be fixing this thing in the next few days, but I dont just want to throw parts at it. Thanks for your opinions! - Mike

EDIT: 1996 Blazer 4x4x4 120k

[MT-2500]

Engine?
Pitch the bosch plugs in the trash can and get some AC -Delco plugs.
If a 4.3 engine you are showing some low fuel pressure.
Also we need the camshaft retard reading.
Any other codes besides missfire?

[mixin]

Originally Posted by MT-2500

Engine?
Pitch the bosch plugs in the trash can and get some AC -Delco plugs.

LOL I knew you were gonna say that, but I was honest anyway!

Yes, 4.3. Is the pressure bleed off to 46 psi after shutoff out of range and indicative of a leak? If so where should I start looking for it or test next?

How do I get the camshaft retard readings? Probably not with a code reader I'm guessing. I need to find someone with a real scanner then?

No other codes according to AZ guy - but then he told me all I needed was plugs and wires! - I wish buddy...

Thanks MT - Mike

[MT-2500]

I would go a set af AC-Delco plugs and see what it does.
Yes you will need a engine capable scanner to get camshaft retard readings.
A slow leak down on fuel pressure is not bad.
Specs are 60/66 on your fuel pressure.
56 engine runiing is a tab low.
Should be around 62 no load.
Go the good plugs and recheck for engine miss first.
Good luck.
MT

[mixin]

Originally Posted by MT-2500

Go the good plugs and recheck for engine miss first.
Good luck.
MT

No prob, will do. But just curious. Could bad plugs cause a cold miss that clears up after awhile? Guess when the ceramic finally heats up, miss goes away?

[MT-2500]

Originally Posted by mixin

No prob, will do. But just curious. Could bad plugs cause a cold miss that clears up after awhile? Guess when the ceramic finally heats up, miss goes away?

Bad plugs or bad plug wires can do a lot of strange things.
And even spark plug gap.
Last week on a Geo I had a customer that replace his plugs and used the factory spec gap on plugs a 28-32.
The engine missed under lugging and had poor gas mileage.
He re-gaped to the new specs on gap setting to 42 and said it made it run better than when it was new and gas mileage came up.

Also Check the latest AC Delco book for plug no changes and gap settings.
A lot of them have been change from factory specs.
Good Luck and let us know how it does.
MT