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1998 K2500 5.7 Hesitation/Misfires


wickedude
02-07-2009, 08:30 PM
Hey guys, looking for some advice on where to go next on my 98 5.7.

At cold idle I dont see any problems, but when warm I get constant misfires on cylinders 6 and 8. When I punch it then the misfires fall to 0 but then I get alot on cylinder 4.

I did a full tune up the other day, changed fuel filter, plugs, cap, rotors, constant 65 pounds of fuel pressure. Swapped ignition coils with a known good coil. No change.Checked CMP retard and it was at 0 degrees.

Truck has been getting bad fuel mileage. Bank 1 has a lean -10.8 STF at idle but bank 2 is +9.9 %. Its seems bank 2 is very lean. Bank 2 sensor 2 stays about 890- 950 millivolts. bank 1 sensor moves around much more. Swapped downstream sensors and get the same rich condition on sensor 2 bank 2.

Pulled new plugs out on 6 and 8 and saw they already had alot of carbon on them. I have not run a compression check yet but this engine only has 53k on it. No coolant in oil or vice versa so Im thinking head gasket is okay. I have replaced the intake manifold gasket already.

Im leaning towards an issue with the fuel injectors or the spider assembly? I dont remember changing the fuel press regulator when I did the gasket. Im thinking of taking of the plenum, cleaning the injectors and swapping to see if that solves it. Can anyone other some other suggestions of what my problem might be?

Thanks!

jonnik
02-08-2009, 06:54 AM
When you said that you have constant 65 PSI of fuel pressure, is that when revving the engine and releasing the throttle? There should be more pressure when throttling up and less when throttling down. Does the pressure quickly bleed down when the engine is shut down? There may be a vacuum leak due to the intake manifold gaskets or a problem with the pressure regulator. If you decide to pull the upper plenum, I suggest changing the regulator (it's under the plenum) and using a good quality gasket set. I don't know if the injectors can be individually swapped or not, you may have to replace the entire spider. I think that most aftermarket spiders have replaced the poppets at the end of the fuel lines going to the cylinders with injectors.

wickedude
02-08-2009, 10:12 AM
What I meant was that it is a constant 65 psi at idle and it does not change greatly under load, like a massive pressure drop. Pressure shoots up to almost 70 under throttle and then backs down. Normal operation with the throttle application.

I had already replaced the intake gaskets awhile back. Ive sprayed with brake cleaner and searched for vac leaks and can find none.

I cant see a reason why a regulator would only affect 2 cylinders. If I have to pull the plenum I may just upgrade to the MPI injection spider. I still think that the injectors or poppets are clogged and clear out under throttle. Im going to take a closer looks at the o2 sensors today. Before I go out and buy the MPI spider does anyone have any other suggestions?

wickedude
02-09-2009, 11:31 PM
Ok a few updates. A few days ago I did the tuneup and the truck ran worse. I went back over it today. Checked timing and CMP retard was at 0. Noticed the rotor was pretty loose so I pulled the distributor and found the drive gear was very very worn. Set the engine back to TDC, installed the distributor again and it seemed to be off a tooth. Dropped it back in and set to 0, misfires are gone.

Im very glad its running better now, hesitation and misfires are gone. I do plan on replacing the distributor, its on its way in the mail. Im lost as to how this could have jumped a tooth and why only bank 2 seems to be running lean?

Im going to drive it around tomarrow and recheck my fuel curves. Anyone else seen this before. The truck drives much better now...

Jeremy-WI
02-16-2009, 06:04 AM
You should not be able to have the distributor off a tooth and still have a cam retard of 0, unless you check without the engine running or without revving the engine over 1000 rpms

wickedude
02-22-2009, 12:49 PM
I got the new distributor in this weekend and got it timed to 0. I still cant figure out what I did before to get it to 0 and run so bad. The new distributor had a aluminum base and sits much tighter on the intake manifold. I didnt realize how worn the old drive gear was until I saw the new gear.

Dropped it in and timed to 0, now runs great and no misfires are reported now. Pickup is much quicker now. I also bought the MPI spider upgrade which ill install new weekend. Any ideas about what the mileage should be with the MPI injection on a 5.7 K2500?

Jeremy-WI
02-23-2009, 06:21 AM
You may not see an increase in mileage if there isn't something wrong with your CSFI system or fuel pressure regulator

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