My gmc sierra has a ghost misfire cannot figure it out

Ryanrsullivan · February 20, 2014

I have a 97 gmc sierra and I cannot figure out this misfire. I bought the truck about a year ago and recently blew a head gasket and came to find out the motor I had was actually a 5.0 bored 30 over roller motor somebody must have thrown in for

Some reason I have no idea, checked the vin and it came with 5.7 from factory

So I buy a brand new 5.7 crate gm motor from jeggs and put it in get it timed correctly and put new headers on with egr delete done brand new exhaust new distributer new plugs, wires, and much more which is irrelevant. Everything seemed fine for about a week

Then I noticed a misfire and trans not

Shifting into o/d which seemed weird and then it went away. And now it randomly misfires and randomly runs fine and is completely random. The only thing I am thinking about doing is changing out the spider injectors but wanted to see what yal thought

Thanks again I'm kind of stuck on this and outside view always helps

Chris · February 21, 2014

Are you running CSFI or MPFI?

Doug_Scott · February 21, 2014

First thing to do is check common stuff before heading into changing injection parts. All those parts are very relevant. And even though they are new, they can still be defective. They come with a warranty for a reason.

Pull distributor cap and take rotor off. Can you see any carbon tracking on the underside of the rotor? Also look for any dampness inside the cap.

Are the plug wires tailor made, or the ones you cut and crimp the end on? Did you actually verify plug gap before installing? Are the wires touching any of the exhaust?

Are all the sensors connected? Have you checked for vacuum leaks? You don't say, so, when does it misfire? Are you using the intake from the 305 engine?

I don't really like the grammar police on some forums, but, that second paragraph you wrote really needs some commas or maybe break it up into smaller sentences. I have hard time understanding what you mean.

Chris · February 21, 2014

I asked the injection question for research and to tell how to check fuel pressure

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Ryanrsullivan · February 21, 2014

First thing to do is check common stuff before heading into changing injection parts. All those parts are very relevant. And even though they are new, they can still be defective. They come with a warranty for a reason.

Pull distributor cap and take rotor off. Can you see any carbon tracking on the underside of the rotor? Also look for any dampness inside the cap.

Are the plug wires tailor made, or the ones you cut and crimp the end on? Did you actually verify plug gap before installing? Are the wires touching any of the exhaust?

Are all the sensors connected? Have you checked for vacuum leaks? You don't say, so, when does it misfire? Are you using the intake from the 305 engine?

I don't really like the grammar police on some forums, but, that second paragraph you wrote really needs some commas or maybe break it up into smaller sentences. I have hard time understanding what you mean.

Ryanrsullivan · February 21, 2014

Ok yea I'm sorry for grammar, when I said irrelevant I was speaking off new starter ac compressor etc. I checked most of the common stuff already such as cap and rotar

The wires are made for my truck and I did verify the gap when installing, as for the wires touching the exhaust I will check again I'm pretty sure they aren't but I do need to invest in wires sleeves since I have headers not manifolds.

I have checked all sensors and they are all connected, the vacuum is fine I redid all the lines when putting in new motor, and the misfires seem to be completely random. It will run fine for 15-30 mins and then start misfireing for 15-30 mins and then run fine again.

As for the intake yes I did reuse it just cleaned it up real we'll only reason being when I verified vin it came with 350 so I figured it was actually made for a 350. When I took the 305 out the intake was black! Thus figuring it had been flooding constantly due to fuel mapping probley never been changed.

I figure the old

Owner blew the original and had a 305 laying around and said **** it. When I got my egr delete the tuner shop said my trim and fuel pressure where good and fuel pump is 5 months old.

I apologize for original post was in bit of a hurry

Thanks

Doug_Scott · February 21, 2014

Is the misfiring at idle, or while driving along at 40mph? What transmission is in the truck? I ask because if the misfiring is at the same time the transmission is in high gear and if the transmission also has a lockup converter, it may not be a misfire doing it. May be the lockup converter "chattering".

Ryanrsullivan · February 21, 2014

Is the misfiring at idle, or while driving along at 40mph? What transmission is in the truck? I ask because if the misfiring is at the same time the transmission is in high gear and if the transmission also has a lockup converter, it may not be a misfire doing it. May be the lockup converter "chattering".

Ryanrsullivan · February 21, 2014

Yes it will do it at idle, you feel it

Much more at idle then cruising speed. Transmission seems fine just won't shift into 4th gear lockout o/d whatever it's called. Oh yeah and it is a 4l60e. Also when hooked up to scanner it says cyclinder 8 is

Misfireing

The thing that gets me is it will run perfect and then randomly won't

Doug_Scott · February 22, 2014

Yes it will do it at idle, you feel it

Much more at idle then cruising speed. Transmission seems fine just won't shift into 4th gear lockout o/d whatever it's called. Oh yeah and it is a 4l60e. Also when hooked up to scanner it says cyclinder 8 is

Misfireing

The thing that gets me is it will run perfect and then randomly won't

4th gear would be OD, and after that you get lockup. I am not sure about this, but, I think the lockup happens in 3rd gear as well. The effect is essentially 6 different effective gear ratios, if you consider the unlocked convertor as a gear.

If cylinder 8 is setting a misfiring code then that pretty much leads you to the spot. Check all the normal things(including compression), don't assume that since you changed it, it can't be that part. I am not familiar with the type of fuel injection on your truck, but, if there are injectors for each cylinder, swap the number 8 injector for injector from opposite side and end of engine. See if the misfire "moves ". Same for wire and plug. Do these one at a time though. No sense changing all three things at one time, that will cost you to replace all three items. Also check for vacuum leak at the intake port to head area. Does the power brake booster vacuum line come off the intake port for cylinder 8?