1998 K2500 454 4x4 Codes - Stumped!

[Big Hands]

First of all, hello. I'm new to this forum and I'm sure like many of you, discovered it because I'm looking for answers to problems I'm having with my vehicle. Thanks in advance for any assistance that may be offered and I look forward to participating here.

My "issue" (this is long, but will hopefully save time in the long run):

I bought a 1997 Chevrolet K2500 4x4 454ci Suburban with about 160K miles on it about a year ago to pull a travel trailer and other wise sit in the driveway and look pretty considering the price of fuel these days.

I took a trip last fall and about 80 miles from home on the freeway, the SES light came on and I pulled off the freeway at which time the vehicle was idling horribly. Spent the night and following morning fiddling around and after buyuing a code scanner (P0300) and trying a few things, figured out that that the cap and rotor had never been changed and looked pretty bad. Changed those and the beast came to life and ran great the rest of the trip.

A month or two later, the SES light came back on and since it was winter and we were not planning any trips soon, we took it into our mechanic who has proven very economical and trustworthy over the past few years since we have been using him.

Initially, it was showing codes P0300, P0161 and P0420 which are random misfire, o2 heater bank 2 - sensor 2, and catalytic efficiency low respectively.

Had all for o2 sensors changed, complete tune up (plugs, wires, cap and rotor), compression check, intake manifold R&R due to leaking anti-freeze and it was still showing the P0300 code and idling rough. He cheked on his scope and found cylinders 5, 6 & 7 were misfiring mostly at idle and cleaning up considerably once it was 150 or so rpm off idle.

Next, he cleaned throttle body, checked EGR and tested after bypassing, cleaned injectors and rail, sent injectors from cylinders 5, 6 & 7 to be tested (all are OK), tested fuel pump which is showing 50psi (possibly that pressure is slightly low - manual calls for 58-62psi), checked idle control motor, fuel pressure regulator and we are running out of things to check.

Still a rough idle and code P0300.

Computer?

Coil?

???

Just wondering if anyone here has any insight into problems like this and how to go about checking and repairing?

Thanks in advance,
Jeff

[Deltadog]

have you done any king of compression test to conferm that compression is good in all cylinders and also check the mas sen.

[Big Hands]

Originally Posted by Deltadog

have you done any king of compression test to conferm that compression is good in all cylinders and also check the mas sen.

Compression checked (as stated in OP), twice actually - 135 to 150psi.

I don't know if the MAS has been checked, but I will find out.

Thanks for the reply,
Jeff