98 Olds Regence: Low compression on 1,3,5,6

oldsmobile
regency

#1

So, a few days ago a co-worker brought me his 1998 Oldsmobile Regency. It has the 3.8l v6 (not the supercharged 3.8 everyone wants.)

His explanation of what happened was, “I was backing into my driveway and noticed the temperature gauge was starting to climb. After I shut it off, I tried to restart it but all I got was a pop and a puff of smoke from the exhaust.”

Upon arrival (trailered,) I gave it a shot of starting fluid to see if the motor was just sharing enough compression to keep from starting easily. There was NO water in the oil.

After doing a dry compression test the results were;
Cyl 1-90psi Cyl 2-150psi
Cyl 3-90psi Cyl 4-150psi
Cyl 5-120psi Cyl 6-90psi

These results are consistent with a head gasket failure. However, having low compression on one whole bank and one cylinder on the other bank was an eyebrow raiser. My next thought was timing chain. I pulled the left bank valve cover and all the rockers are moving.

As of this point, I know the timing chain is NOT broken, but what are the chances of it jumping time?

Side note, I know the car well as I am his regular go-to mechanic, and it has never had signs of missing on any cylinder. That’s why I find it quite odd that both banks show dead cylinders simultaneously.


#2

After shooting starting fluid in the intake, I got a big backfire through the intake which doesn’t happen very often on fuel injected motors like this one.


#3

That GM engine has a wasted spark ignition system.

Which means the ignition coil fires two spark plugs at the same time.

Only one cylinder has air/fuel charge which fires that cylinder and the other does not so nothing happens. Thus wasted spark.

So if you shoot starting fluid into these engines, and the wasted spark occurs in a cylinder where the intake valve is open, the engine will backfire.

I’ve seen plastic intake manifolds explode when this occurs, when a fuel pressure regulator leaks.

You can check if the timing chain is stretched.

Turn the crankshaft by hand in one direction while watching the rocker arms.

Now turn the crankshaft in the opposite direction while watching the rocker arms.

If the crankshaft can be turned 5 degrees or more before the rocker arms begin to move, the timing chain is stretched, and may have jumped time.

Tester


#4

Thank you @Tester,
I have a feeling that’s what has happened, and I will check it out the next time I’m at the shop.
I’ll post what I find out!
-Dustin


#5

I would suggest that you go back and perform a wet compression test. This means a squirt of oil in each cylinder before the compression is checked. If the number jumps up a fair amount over the dry one there’s a problem.

I think you’re going to find this is a worn piston ring issue and possibly exacerbated by the overheating.


#6

@ok4450 forgot one word. Age will do that.

This means a squirt of in each cylinder before the compression is checked.

This means a squirt of Oil

Yosemite


#7

Corrected, Must be age or drugs… :slight_smile:


#8

Thank you everyone,

I have determined the timing chain is not the issue. After checking the timing chain, I happened to notice my compression gauge is missing the o-ring that seals it to the head when screwed in… I will be re-doing the dry compression test as well as a wet test as soon as I find a new o-ring for my compression gauge. I’ll reply with the results later in the week.

Thank you @ok4450, @Yosemite, and @Tester
-Dustin


#9

I got a new o-ring on my gauge last week and ran compression tests again. The o-ring must’ve been gone before I even started because, now the entire left bank is good and it’s the right bank that has two cylinders down. I’ve already started tearing it down for a new head set and all will be fine as soon as I get the pesky exhaust manifolds off.


#10

I find it easier to unbolt the exhaust pipe fron the manifold and remove the manifold with the head.


#11

I agree with oldtimer_11 and seeing as how you have 2 cylinders down it sounds like you’re assuming the problem is in the valves. Maybe not. That’s the purpose of the wet compression test; to determine if the problem is in the cylinders or cylinder head.


#12

Three head bolts are under the exhaust manifold, the exhaust manifold must be removed first.


#13

Didn’t know that, my only experience with GM cars is limited to 2 cars and that was 2 too many. I was spearing generically about removing heads here in the rust belt and find it much easier to seperate head and exhaust manifold on my workbench where it is warm and well lit rather than on the car. Much easier ti tease the nuts off without breaking a stud. Sometimes the stud comes out instead but that is ok too.


#14

I approach most older exhaust manifolds with a great degree of trepidation. Broken studs are bad enough but I’ve had my fair share of difficulties and what seems to happen more often than not is that the manifold changes shape enough with repeated thermal cycling that it’s very hard to reinstall it once it is removed. It’s under stress and once pulled, it “sproings” and takes a lot of patience and messing around to get it back in place.


#15

That didn’t help . . . ?


#16

Yes the problem is in the heads. Unfortunately, as @Nevada_545 said, you just simply can’t get to the outer head bolts without pulling the manifold but, I agree it is much easier to remove the manifold and head together @oldtimer_11. The easiest way to remove the manifolds is to pull the entire studs using 2 nuts cinched together. I’ve always used that method and only had one stud twist off. Another little trick to the trade is to tap the stubborn studs a few times with a ratchet to loosen them up. Steel bolts in an aluminum head usually react very well to this.

@TwinTurbo, I usually sequence the exhaust manifold bolts from middle to end when reinstalling to avoid any deformation problems. Also as I said I’m pretty partial to removing the studs so you don’t have to have every bolt hole perfectly aligned when you start.

Thank you everyone for your responses. I have the exhaust off and the heads are coming out today.


#17

Well, I ask again, Unless you’ve done a wet compression test what is the next step if you redo the cylinder heads, reinstall them, and discover the compression is still low due to a ring problem?


#18

I did both tests after I got the new o-ring for me gauge @ok4450. I do rebuilds pretty often. I only created this post because of the unusual combination of compression numbers which I found out was because of the missing o-ring. Thank you for your concern though!