1998 Chev K2500 - 7.4L Miss Under Load FIXED!!!!

[Tim Martin]

My feeling is that it's not a timing chain issue. I have changed timing chains way too often in situations where the issue lie elsewhere. A chain has to be loose enough to be able to jump a tooth to create a noticeable difference in engine performance.

[protraxrptr17]

heres a secondary pattern during the misfire

[steven kiser]

when you say under load is it when driving or will it do it if you torque it up while braking it? if i remember (fog moving a bit) when i was dealing with misfires on there i would unplug the plug on the transmission that would throw the converter into lock up. if i'm recalling properly it was sometimes difficult to tell the difference between a ship and a shudder. if it will not do it during a power brake it may be a converter shudder. i've had them convince me it was an ignition issue. did you look at the plugs for gap or porcelain cracks. i've seen a fouled plug issue duplicated by a cracked insulator in a replacement set.

[protraxrptr17]

Its a manual transmission. Its a definite misfire. It will cough sometimes. It will also do it if you accelerate lightly. About 2000 to 2200 rpm it will miss and sputter. I cant believe I cant find this. Normally this is my strongest area.

[steven kiser]

can't believe i over looked this. it sounds like a worn camshaft or bent push rod. i've seen this before. not sure on this but, i would pull the drivers side valve cover off and watch for the valve throw. especially the middle two cylinders. can't really nail this down mentally but since i though of this i'm getting more and more confident that you'll find the problem lies there. be careful on this truck. we both know that after the customer gets it back it's going to cost more and more money to fill it with fuel and somehow it's going to be your fault. for some reason people can't get mileage and cost comparison in perspective. since it cost more to fill the tank the gas mileage is off. personally i'm getting tired of explaining it. i feel like rickie riccardo. i have some splainnnning to do.

[Jeff @ Able Auto]

A little off subject here but...I been seein a lot of the "fuel mileage" problems also due to the cost of gas. Basic math aint so basic I guess!

[steven kiser]

A little off subject here but...I been seein a lot of the "fuel mileage" problems also due to the cost of gas. Basic math aint so basic I guess!

i agree, i'm pretty specific when talking about mileage. i ask right away if the are talking about cost or volume. there are some people out here that can't grasp the difference. i had one customer i wanted to grab and shake them into understanding. i had snow birds return and come in complaining that their car had really dropped in fuel economy. after a few minutes of tense discussion it came down to before we left for florida it cost 35 bucks to fill the tank, now it costs 50. well it was clear that the mileage had gone to the crapper. i took a cup and filled it with water. i said today the water cost a buck to fill this cup. tomorrow the water is going to cost a dollar 50. i explained it's the product cost not the volume. they asked what that had to do with the fuel economy. i replied "green ford". they wanted to know what that had to do with the fuel economy. i told them as much as cost to volume does to compute fuel economy. that they understood.............,...

[protraxrptr17]

Think I found it. Lifters were sticking, which was holding the valves open slightly. It was hard to find because there was so much wrong with this truck. New lifters are coming. Maybe I'll get em put in tomorrow.

[protraxrptr17]

Found it on the scope. Number 6 and 8 had a difference in the secondary pattern during the misfire. It was hard to see in the screenshot I posted. I changed the scale to 15kv and it showed up nice. I'm not good enough with secondary patterns to pick out a problem. I have to look for variations between cylinders. I changed the 02 sensors and then I could see by studying the 02 patterns that the right bank was going rich. I couldnt pick it up with the old sensors. I guess they were too slow. Thats when I went back to looking at the secondary.

[steven kiser]

lifters sticking? if they are over pressurizing causing the valves to float i've never seen it happen to a selected few. i've seen worn guides or a lifter journal being oblong. this is usually accompanied by a ticking. at this point i would take off the valve cover, remove the rocker arms, do a leak down test and while the cylinders are pressurized wiggle the valve spring back and forth. any movement isn't acceptable. you may find that there is pressure leaking between cylinders. if this is the case use a flat bar on the block. i've seen more than a few where the block deck has been damaged. if the block is good i replace the head. the first few i did i had the heads reworked only to have issues within a short time. again i'll mention that i've settled on alabama cylinder head after years of using others.

[protraxrptr17]

Already had the heads done. It only leaks when the rocker arms are tightened down. Number 6 only had 60 LBs compression. Loosen the rocker arms and it only had 10% leakage. Tight it would go to 80. Only thing that it can be.

[protraxrptr17]

If you look at the scope pattern I posted you can see the problem cylinders as 8, 6, and 2. Of course its easy to see after the fact.

[ricmorin]

Already had the heads done. It only leaks when the rocker arms are tightened down. Number 6 only had 60 LBs compression. Loosen the rocker arms and it only had 10% leakage. Tight it would go to 80. Only thing that it can be.

Whoah! The heads were off? When did you do that?

[protraxrptr17]

Its in my first post.

[Tim Martin]

Isn't the push rods on this engine different length between the intake and exhaust? I think they are. What's the possibility if you had the heads off and got the push rods mixed up for those cylinders when you put it back together again?

Just a thought.

[liljoe]

I would be looking at the valve stem height real close and make sure that you don't have a couple that are too tall.

[Tim Martin]

I would be looking at the valve stem height real close and make sure that you don't have a couple that are too tall.

Well I thought of that too but a good machine shop will check that. Somehow I do not see stuck lifters as the source of the miss in this situation. Not that it never happens but ...........

[steven kiser]

i missed the valve job as well. i didn't look at the putting it back together as a valve job. my bad, i guess. another point here is even a great machine shop makes errors just like we do. if the valve stem wasn't trimmed to proper height the valve would definitely hang open when the lifter was adjusted to eliminate and lifter clatter. the spring tension would be off as well. this may not initally cause an issue but later in it's life may slow the return enough to cause a sputter. i'll remind abot my post on specifics.

[protraxrptr17]

Exact same symptoms as before the valve job. No better,no worse. We will see. I'm doing the job right now. These rockers are not like the old style. These have shoulder bolts. No adjustment. It's all done at the lifter.

[steven kiser]

what about pressure leak from cylinder to cylindermany, many moona ago i mis-read a leak down test because i was charging the next cylinder as well and together they held pressure but stand alone with plugs out the didn't