92 5.8 Low compression
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1st Gear
Joined: Dec 2023
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92 5.8 Low compression
Hello all. Just wanting to get an opinion or 3. I have a 92 with a 351 w/ 219k miles, all stock. About a year ago, just started running crappy outta nowhere. Misfiring condition combined with shuddering shifts, especially 3-4. Haven't driven it much at all since. Finally about a month ago, I decide to look deeper. Did a compression test. I'm 120-130 across the board.... except #1. It's at 30psi. So then, i put just a drop of oil onto the top of the cylinder and retested the compression...still at 30-32 psi. If I'm correct, if the pressure would have went way up, that would mean the rings would be shot? So am I looking at the valves now?
How easy is it to pull the heads off of these things? I've never worked on one before. I'm no mechanic by far, but not afraid to turn a wrench, as I've owned and worked on several Turbo Coupe Birds & done a lifter job on an LS motor.
And i guess the secondary question behind all of this is, since it looks like I'll be taking the head(s) off, what would be things to do or any good supporting mods to do while in there?
How easy is it to pull the heads off of these things? I've never worked on one before. I'm no mechanic by far, but not afraid to turn a wrench, as I've owned and worked on several Turbo Coupe Birds & done a lifter job on an LS motor.
And i guess the secondary question behind all of this is, since it looks like I'll be taking the head(s) off, what would be things to do or any good supporting mods to do while in there?
Last edited by Eff1Fiddy; Dec 26, 2023 at 06:18 AM.
Logistics Pro
Joined: Aug 2010
Posts: 3,716
Likes: 583
From: Chillicothe
Well cylinder one is a concern definitely. But 120 psi is not the worst i have seen, are you doing this hot or cold engine? Are you doing the compression check with the throttle wide open?
If I were you. I would knock the guts out of a sparkplug and weld a air chuck to it and do a leak down check. To see if the air is going passed the rings or out a valve.
Or buy a leak down tester from harbor freight or amazon, they are relatively cheap.
This does a couple things.
1. Tells you right off if it is a ring or valve problem
2. Keeps you from doing extra work ( pulling a head in the truck, if the whole engine needs to come out)
If it is just a valve problem, you may get by throwing a used head on it and get back on the road.
If I were you. I would knock the guts out of a sparkplug and weld a air chuck to it and do a leak down check. To see if the air is going passed the rings or out a valve.
Or buy a leak down tester from harbor freight or amazon, they are relatively cheap.
This does a couple things.
1. Tells you right off if it is a ring or valve problem
2. Keeps you from doing extra work ( pulling a head in the truck, if the whole engine needs to come out)
If it is just a valve problem, you may get by throwing a used head on it and get back on the road.
Laughing Gas
Joined: Oct 2014
Posts: 946
Likes: 186
From: Carnation, Wah
Drive it a bit and recheck compression on #1.
It's not uncommon for something stuck to a valve seat-
often, carbon chunks from pulling a plug-
and that'll mess with the numbers.
#30 is too high for valve damage- once a valve loses seal, combustion pretty quickly burns it to the
point where it won't build pressure at all.
t
It's not uncommon for something stuck to a valve seat-
often, carbon chunks from pulling a plug-
and that'll mess with the numbers.
#30 is too high for valve damage- once a valve loses seal, combustion pretty quickly burns it to the
point where it won't build pressure at all.
t
Hotshot
Joined: Jul 2020
Posts: 10,349
Likes: 2,588
From: Near Salt Lake City
Pull the valve cover
Inspect the valves and their movement for cylinder 1 compared to the rest while cranking over or running if you put it back together
Make sure one of the valve springs is not broken or the pushrod bent
Shoot shop air into the #1 spark plug and check for the valves leaking with number 1 in firing position TDC
See if the shop air injected into cylinder 1 comes out the other valve cover breather (indicating a piston or bore issue)
If it shoots a bunch of air out the intake or exhaust, then pull the heads off and inspect the valves
Inspect the valves and their movement for cylinder 1 compared to the rest while cranking over or running if you put it back together
Make sure one of the valve springs is not broken or the pushrod bent
Shoot shop air into the #1 spark plug and check for the valves leaking with number 1 in firing position TDC
See if the shop air injected into cylinder 1 comes out the other valve cover breather (indicating a piston or bore issue)
If it shoots a bunch of air out the intake or exhaust, then pull the heads off and inspect the valves
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