Need advice about low compression

[funky at best]

Hey all! I'm new to this forum stuff, so sorry if I do any of this wrong.

So here it is. I got a '91 7.3 IDI. I noticed a "puffing" coming from the exhaust at idle, so I did a compression test. I had 420lbs on 2 cylinders, 390lbs on 4, 360 on 1, and 200 on the last one. I understand these motors have a habit of rod guides wearing out. I'm assuming for now that that is it.
Here are my questions;
1. is the range of the other cylinders (360 to 420) bad, or normal?
2. How difficult a job is it to pull the head(s) and do the valve work at home? I have never done any type of engine rebuild, although I do all my own maintenance and almost all repair work, and am very mechanically inclined. I can rebuild carbs and that sort of thing, but the farthest I've ever gotten on a motor was the intake manifold. I'm sure of course I'll have to take the head to a machine shop to get milled and have the valve guides pressed and stuff, but I'm wondering if I should just take the whole dame thing in and let a pro handle it.
3 Does anyone have a machine shop in the Santa Cruz Co./Bay area that they trust?

I've had bad luck with shops, and don't really trust them. But having no rebuild experience, I am worried about getting in over my head and taking a freaking month to do a job that a shop could do in a day or two!

Thank to all in advance!

 

[laserjock]

@towcat @IDIoit

These guys are in your area. Sounds like a valve not seating. Could be because of a guide. You probably don't have to lift the head to check the guides.

 

[franklin2]

Rebuilding the heads and repairing the valve guides is specialized work. I am sure you could do it if you had the machines that cost thousands of dollars. The head has to be setup on the machine so the guides are lined up, and then they need to be bored out for new guides or brass sleeves. Then they have to be finished to size for the proper clearance to the valve stems. After all that, the valves and the valve seats have to be ground so they seal to each other.

 

[riotwarrior]

Zearch for my post mortem video on chuff out exhauzt......

 

[IDIoit]

the first thing I would do is take that compression tester and throw it as far as you could.
the next step is to do a leak down test.
a leak down test will tell you exactly where your issue is.
intake or exhaust valve, rings, or head gasket(somewhat)
this gives you the pointed finger in the right direction.

this could be a day or two job, but you would have to have a set of rebuilt heads ready to slap on.
if the issue isn't with a valve, you gotta dig deeper.

 

[BDCarrillo]

I used my cheapo Chinese compression tester fittings to put ~20 psi into my bad cylinder, and an -on head valve spring tool to take spring pressure off. I was then able to deflect the valve a bit by hand and hear the air leaking into the exhaust. I also had a dial gauge set up on it, but don't remember how much deflection I got from a bad vs good valve. Give that a go and you can narrow it down.

 

[riotwarrior]

FYI

IF....you remove valve springs have some Fat O RINGS sized for the small diameter of vqlve stem where the locks sit...carefully slip it on valve.....if valve drops it shouldnt go all way into bore....Just a thought to consider if piston decides go down in bore due to air or valve wont seal due to wear etc....

Err on caution

 

[Knuckledragger]

the first thing I would do is take that compression tester and throw it as far as you could.

Disagree. You already know that one cylinder is different from the others, that will be where you focus your investigation. A leakdown is handy to pinpoint the problem, but not crucial. Taking off the valve covers might also tell a lot. The valve train in these engines is pretty good, but does have problems from time to time. Bent valves, worn guides, collapsed springs, broken rockers, etc. are all possible.

300-420 psi is just fine for an N/A engine. It is a pretty wide gap (you don't usually want more than 10% difference), but the lower numbers have sufficient compression to do the job.