93 Suburban runs rough at idle,loss of power

My '93 Sub. with a 350 and 178,000 miles is running rough at idle. Seems to be more noticeable when in gear. Doesn't really seem like a miss, but not saying it isn't. Also power on acceleration seems down.

I have replaced plugs, distributor cap, rotor, and PCV valve. I ohmed out wires with the highest being 16,000 ohms on the longest. I doubled checked the routing of the wires and they are not crossed. The catalytic converter wasn't glowing red tonight when I got home. The fuel pump was replaced about 80,000 miles ago as preventive maintenance. Pump still sounds the same as when it was replaced.

I'm looking for thoughts on what to check next. I have not done a compression check. Would need help on how to disable fuel system if I do this next.
Thanks

My '93 Sub. with a 350 and 178,000 miles is running rough at idle. Seems to be more noticeable when in gear. Doesn't really seem like a miss, but not saying it isn't. Also power on acceleration seems down.

I have replaced plugs, distributor cap, rotor, and PCV valve. I ohmed out wires with the highest being 16,000 ohms on the longest. I doubled checked the routing of the wires and they are not crossed. The catalytic converter wasn't glowing red tonight when I got home. The fuel pump was replaced about 80,000 miles ago as preventive maintenance. Pump still sounds the same as when it was replaced.

I'm looking for thoughts on what to check next. I have not done a compression check. Would need help on how to disable fuel system if I do this next.
Thanks


Check for a cylinder miss.
Good hot blue spark tom all plugs?
Is the egr valve sticking open?
What is engine vacuum at idle and speed up?
Check for proper fuel pressure.

Thanks MT. I will not be able to look at it for a few days. Plugs all looked to have had good color, nothing to lead me to any one cylinder.

After installing items in first post i took it out and drove it. Got on it kind of hard. Wife called when driving it to work this morning and said she heard a knock. I didn't hear it yesterday, hope I didn't throw valve timing off somehow?

Thanks MT. I will not be able to look at it for a few days. Plugs all looked to have had good color, nothing to lead me to any one cylinder.

After installing items in first post i took it out and drove it. Got on it kind of hard. Wife called when driving it to work this morning and said she heard a knock. I didn't hear it yesterday, hope I didn't throw valve timing off somehow?

A engine vacuum reading will tell you a lot of things.
Post back redings.

For engine miss pull one plug wire at a time and see if rpm drop on each cylinder.

I had time to short out plug wires and found cyl 4and 6 not to be firing. Tried different wires and plugs to make sure they were not bad. No luck. Will do compression test next, but leaning toward head gasket now. Don't know what else the two have in common. Might take a week to post back due to work.

I had time to short out plug wires and found cyl 4and 6 not to be firing. Tried different wires and plugs to make sure they were not bad. No luck. Will do compression test next, but leaning toward head gasket now. Don't know what else the two have in common. Might take a week to post back due to work.

Make sure plug wires are not crosed and good hot spark out of plug wire.
Pull plugs and check plugs.
Wet or dry on end?
Run a compression test while plugs are out.
Post back what you find.

Quick update on what I found
Compression check cold
Cyl 2 172 psi Cyl 1 175 psi
Cyl 4 176 psi Cyl 3 65 psi
Cyl 6 170 psi Cyl 5 76 psi
Cyl 8 178 psi Cyl 7 175 psi

Pulled head and comfirmed head gasket blowout between 3 and 5
Valves will be touched up as a couple have slight leakage when I sprayed brake cleaner on top side and drippled by them.

Opposite head will also come off as I am this far.

Quick update on what I found
Compression check cold
Cyl 2 172 psi Cyl 1 175 psi
Cyl 4 176 psi Cyl 3 65 psi
Cyl 6 170 psi Cyl 5 76 psi
Cyl 8 178 psi Cyl 7 175 psi

Pulled head and comfirmed head gasket blowout between 3 and 5
Valves will be touched up as a couple have slight leakage when I sprayed brake cleaner on top side and drippled by them.

Opposite head will also come off as I am this far.

Thsanks for posting back what you found.
I would get the heads check for warpage while you are at it.
Also new head bolts when you put it together.
Good luck

Truck is back together. Opposite head was near the point of blowing gasket on center cylinders also. Well worth time pulling apart. Heads and block decked out with no warpage. Dressed up valves also.

Thought pulling it apart would be the tough part, but found going back together was not that easy. Exhaust manifolds were next to impossible to line back up due to warpage, rust in power steering pump shaft had me head head scratching until I ran a tap in it to clean up the threads so I could reinstall pulley.

Don't want to think what it would have cost for a shop to to this repair.

You're lucky dude, I have a 94 with 2 dead cylinders. We were hoping that it was just a head gasket. So my dad and I tore down the engine. Once the heads were off, we saw that the head gasket was obliterated between cylinders 4 & 6. Unfortunately, once we started to clean the head, we found a nice size crack. We were like, oh well, we'll just get new heads. Then we went to clean the block, that's when we realized it was a lost cause. There was a crack in the cylinder wall right where the gasket was missing. So that engine is officially dead.

And yeah, for the price, you might as well buy a new engine, or a new truck.