Compression test results - now what?

[bumanfam]

1970's GM 165 L6 250 - I finally got fuel to and through the fuel pump and through the (now rebuilt) carb. Got the engine running for the first time!

Started to go for tune-up, following the steps outlined in my service manual. Plugs cleaned & gapped to specs, and on to compression testing.

1 - 110
2 - 115
3 - 105
4 - 120
5 - 0
6 - 0

No service techs here - I'm doing it all myself until its good or I have a bunch of parts to sell! I'm mechanically able, but wholly untrained, so it's me, a service manual, and y'all!

So, the question is, what should I check next?

 

[JustJason]

Re: Compression test results - now what?

you can do a leakdown and find out where your compression is going.... or... check your valves and make sure they are opening and closing and the lash is set correctly

 

[HT32BSX115]

Re: Compression test results - now what?

You probably have a somewhat worn out engine, possibly with a couple of sets of burnt or stuck (open) valves, cracked or "holed" pistons and /or other problems.

One of your options might be to go to a wrecking yard and just get yourself a 70's-80's 250 cu-in truck engine. Just ensure that the head is the same and your engine mounts will bolt up to the block.

The 230/250/292 engines are probably some of the BEST engines ever used in marine applications. The 230 cu-in 150hp Mercruiser I had in my previous 1966 MarkTwain (dad bought new) ran 39 years without even the head being removed!! The hr meter broke in the mid 80's at 1500 hrs and I sold it in 2005. The only major failure was the coupler after 38 years!!

1970's GM 165 L6 250 - I finally got fuel to and through the fuel pump and through the (now rebuilt) carb. Got the engine running for the first time!

Started to go for tune-up, following the steps outlined in my service manual. Plugs cleaned & gapped to specs, and on to compression testing.

1 - 110
2 - 115
3 - 105
4 - 120
5 - 0
6 - 0

No service techs here - I'm doing it all myself until its good or I have a bunch of parts to sell! I'm mechanically able, but wholly untrained, so it's me, a service manual, and y'all!

So, the question is, what should I check next?

 

[JustJason]

Re: Compression test results - now what?

sure you can leakdown a motor with 0psi....
why CANT you???

 

[Uraijit]

Re: Compression test results - now what?

You can do a leakdown test, and it's a good idea. Do it, and you'll know where it's leaking. If you've got ZERO compression, I'll bet you a buck it's a bad valve, but it's easy to figure out. You can just blow compressed air into the dead cylinders (through the spark plug hole) and listen to where it comes out. No fancy equipment necessary.

The next step is to pull the head, and see what's up. If it's just the head that's got issues, there's no reason to go buying another engine (junkyard or rebuilt). Just pull the head, find out what's wrong, and go from there.

If it does in fact need a rebuild, the gaskets aren't really that expensive. I just rebuilt and marinized a junk yard SBC 350 with Summit Racing parts.

The gasket set was $40, and included ALL gaskets (including a Fel-Pro composite head gasket set) as well as, main seals, valve seals, etc... Here's a similar set for your engine, for $53 (a little more than mine was, but still damn reasonable) http://store.summitracing.com/partd...840131+4294819503+4294925237+115&autoview=sku

Rings were $20.

The brass plugs were another $16.

Spent another $40 or so because I busted a head bolt, and bought a new bolt set and a replacement head gasket at Autozone.

Oh, and the engine cost me $69.99 at the salvage yard... It was a GM Goodwrench Crate motor. When I opened it up, the cross-hatching was still untouched, which lead me to believe that the engine was not yet broken in. I'd already bought the gasket kit with new rings, for dirt cheap, so I went ahead and re-honed it, and replaced the rings while I had it torn down... It was a truck engine, so I didn't bother with the cam. If I have problems, the cam will be easy to upgrade later.

All told, I put about $189.99 in it, if you don't count all the oil and filters and other incidentals that you'll need whether you rebuild or buy new.

$190 is a FAR cry from $1000+.

I did all the work in 4 evenings in a friend's garage (had to borrow his, cause I don't have one). The hardest part was finding the time to drive the 40 miles up there to work on it. I probably spent more money on gas to drive up there, than I did on the whole engine!

 

[thumpar]

Re: Compression test results - now what?

If you can buy a new Chevy Crate Engine for $70 I need to start shopping at your junk yard!!! Heck, a 350 block alone is worth $100 to $200 to engine rebuilders.


By the way, can you describe how you do a leak down test when the cylinder has zero PSI in a compression test? How do you leak below zero?

We have a junk yard here that sells any engine they have for $100. I have a few friends that own mustangs that go weekly to find ford 302 motors, they go quick.

It wouldn't be a leakdown test in the normal fashion. It is more to just find out there the air is going.

 

[Uraijit]

Re: Compression test results - now what?

Actually, I misspoke, I only paid $65 for it. Wednesdays are "Half-Price day". It's usually $129 for a complete engine. If you just need a short block, it's normally $99 ($49 on Weds)http://pnsauto.com/pricelist.html

Check your local salvage yards, a good deal of them have "half-price days", and sometimes they'll have a half-price special for holiday weekends. It's 4th of July weekend afte rall!

As I said before, just take some compressed air, and blow it into the dead cylinders, and listen to where it's coming out. It's not a "leak-down" test in the sense that you're waiting for it to leak down, it's more of a "leak" test that lets you hear where it's leaking from.

If it's coming out the exhaust manifold, you've got bad exhaust valves. If it's coming out the intake, you've got bad intake valves. If it's coming out the oil dipstick tube, you've got other problems.

There's also the possibility that it's just a badly blown head gasket, or a small possibility that there's a busted cam. If the cam's busted, though, the distributor won't be turning.

 

[Uraijit]

Re: Compression test results - now what?

Oh, and it wasn't a "brand new crate engine" in that it was still in the crate. It was a crate engine that someone had put into a Blazer, and for whatever reason, scrapped before the engine was broken in. The engine was like-new inside, but it wasn't LITERALLY new in the crate. Read my post.

I'm guessing something went wrong with the 4-WD or something major, and they decided that it wasn't worth dumping any more time/money into, and got rid of it. I got lucky on this one, but it's not hard to find a block in good shape -- especially the SBC...

 

[JustJason]

Re: Compression test results - now what?

Can you explain how you would do that? I mean I have rebuilt engines with busted/missing rings or holed pistons and still have SOME pressure when doing a compression test.

sure.... hook up your leakdown tester:
http://buy1.snapon.com/products/diagnostics/eepv309a.asp

turn on the air and listen for the leak.... if you have 0psi your only going to hear it from the intake side, the exhause side (possibly both) or in the oil pan.....

If you don't have a leakdown some compression testers have leads that can be modified by pulling a scrhader valve out of them and then hooked up to the shop air.... if your going to do that just don't put 120 pounds into a cylinder... 70 or 80 outa do it.

If your getting 0 psi... (aside from a missing piston altogether) your valves are probably sticking or not opening..... have you pulled the valved cover and checked the lash like i stated in post #2 yet?

 

[bumanfam]

Re: Compression test results - now what?

sure.... hook up your leakdown tester:
http://buy1.snapon.com/products/diagnostics/eepv309a.asp

turn on the air and listen for the leak.... if you have 0psi your only going to hear it from the intake side, the exhause side (possibly both) or in the oil pan.....

If you don't have a leakdown some compression testers have leads that can be modified by pulling a scrhader valve out of them and then hooked up to the shop air.... if your going to do that just don't put 120 pounds into a cylinder... 70 or 80 outa do it.

If your getting 0 psi... (aside from a missing piston altogether) your valves are probably sticking or not opening..... have you pulled the valved cover and checked the lash like i stated in post #2 yet?

I haven't pulled the cover yet. Last night (after being excited to get the thing to run, however bad) when I did the compression test, I had just enough light to clean up my tools. Today's been unavailable to me.

Lots of great info and suggestions here, guys, and I thank you for that. I think my first move will be to have a go at locating the airflow direction (and hope for a valve), then pull the cover to see what's going on.

It's all new to me, but I'm having a blast. I did tell the wife to take all the "boating accessories" back to the store - I may need that cash and we're not likely to get arount to it for a while yet. . .

I'll post what I find.

A follow-up ? though. I did get an occasional backfire during my brief run yesterday - would a leaking exhaust valve allow mixture out to the manifold, later to ignite or am I an idiot?

 

[JustJason]

Re: Compression test results - now what?

backfire through the carb or through the drive?

 

[Uraijit]

Re: Compression test results - now what?

Yes, backfires point even more toward a valve issue (stuck or burned).

 

[Fireman431]

Re: Compression test results - now what?

Pull the valve cover and spin the engine over. Make sure the valves are opening & closing like their supposed to. Stuck lifters or valves are easier to fix (wishful thinking).

 

[bumanfam]

Re: Compression test results - now what?

I still say all the testing is still a waste of time.


Assume it has a bad intake valve. - Pull the head.

Assume it has a bad exhaust valve. - Pull the head.

Assume it has a bad head gasket. - Pull the head.

Assume its a holed piston. Rebuild which means, yup, pull the head.


Anyway you look at it, the head needs to come off and you may as well get it done so you can see what is happening.

Okay, okay! I'll pull the head already! Looking at it, seems I can do that without pulling the engine first, am I mistaken?

 

[pensacola]

Re: Compression test results - now what?

You really need to read up on that manual.

 

[bumanfam]

Re: Compression test results - now what?

Some Repairs, Listed in This Section, Must be Completed with Engine Removed from Boat. Engine Removal Depends Upon Type of Repair and Boat Design.

Many tasks require the engine to be removed from the boat. If engine removal is not necessary, the reader may still elect to remove the engine in order to perform the work more easily.

Real conclusive. Yep, I have both manuals. I did not make my boat model clear, however, when aksing if I must pull the entire engine first. It's a 1974 Century Raven 17. Now, know any reason I can't pull the head off without pulling the entire engine first? I see none, but appreciate your sage advice.

 

[bumanfam]

Re: Compression test results - now what?

Boy, was this a long time ago! I did pull the head - it was a valve problem. And a piston problem. And once I got the exhaust manifold out of the way, I saw the huge freeze crack. That's where I quit. Interest has waned dramatically. I'd call the whole boat an anchor, but it still floats! Anyone want a project for the winter?