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Urgent Challenge: Should I Remove the Head?

Im trying to determine the cause of my engine failure and why Im only getting almost exactly 30psi’s compression on EVERY cylinder:

GM 151 2.5L (Vin-R) 4-Cyl.Engine

90,000 Miles on New Eng., Reg.Oil Chgs.

1. CYL. 1-2-3-4 (apx. 30 psi’s ±5psi) Normal is at least 105 psi’s

2. OIL - Clean

3. Water - Clean

4. Timing - Correct (Checked TDC on Cyl#1)

5. Rocker arms moving on all cylinders

6. Valve Cover removed inspected lifters all in good cond.

7. Oil drained through fine mesh and no significant metal particles found (1/10th gram).

8. Oil Pressure at time was unknown, no oil gauge or warning light displayed at time of failure.

Recent Background: Vehicle had overheated hours earlier due to known slow-external leak in cooling system. Outside temperature at time was apx. 25 deg. (F)Replaced coolant with water until permanent repairs could be made to leak. Vehicle driven afterwards with no further problems for a couple short trips (10 miles). I have limited tools and money and will have to buy tools/gaskets if I take the cylinder head off for inspection. I have over 25 years working on cars, rebuilding engines, but based on the “EVEN” low-compression on ALL cylinders and other mentioned facts, Im stumped on this one.

A. If by chance I lost a cylinder bearing (exposing an oil port) could that in any conceivable way account for precisely the same cylinder pressure?

B. Is it possible, even likely, that I could bust a head gasket ONLY in the spots adjoining the four cylinders and not in any water/oil ports (it would have to be TWO separate adjoining spots)?

C. Is it more likely, over all other improbabilities, that my newly purchased Harbor Freight compression tester is a “cheap piece of junk” and giving me false readings, explaining why I get even pressure on all cylinders?

D. In spite of the adverse cost to me, should I still take off and inspect the cylinder head, or is it the problem likely elsewhere?

I have to get on this while the weather is good for a limited time, so I have to make decisions on courses of action ASAP. Thanks in advance for your replies.

The only way I know that all four cylinders can drop dramatically the same amount is if the valve timing was off.

Before tearing into the motor, have another mechanic use his own compression tester which should confirm your new tester as either accurate or a piece of junk. Too coincidental and unusual to have all 4 cylinders with the same reading. Confirm the accuracy of your tester before trusting its diagnosis.

Did you have the throttle wide open when you were cranking the engine in order to get the compression readings?

If not, redo your compression test.

Also, do the compression readings go up when you add oil to the cylinders?


Thanks so far, but I encourage more replies.

I got an idea I’m gonna try first. Im going to leakdown several ounces of oil in cylinders 1 and 4. If after plugging them and cranking I find that same oil in 2 and 3, I have my unlikely answer! The gaskets broken between all cylinders. If negative, I will still re-test compression.

As to other replies, I simply dont have the budget to tow the vehicle back and forth to a mechanic for compression verification. I’ll see if I can find a nice one to try my tester on a good engine when I can find a nice one. Yes, I wanted to believe it had to be the timing but the TDC mark-to-mark and on cylinder# 1 doesnt lie. I preferred to believe this as the cause due to the other compelling facts but no. Also, this 87 Century doesnt have a carb, it has a throttle-body with fuel injection so I cant “open-throttle” this vehicle like a carbureted one.

Again, please keep coming with your ideas and thanks.

It’s throttle body injected rather than carburated, but it still has a throttle. And it still needs to be open. If not, the throttle will act as a restriction, interfering with the free flow of air into the cylinders, and lower the compression readings.

You can open the throttle at the pulley where the accelerator cable opens it.

That engine uses a timing gear and it would be unlikely that it would jump time but the cam gear is prone to strip which would stop the cam from turning and more than one cylinder would have virtually zero compression. Watch the rockers while the engine is cranking and confirm that the cam is turning. If turning loosen the rockers and pressurize the cylinders to find the leak. It is likely to be a bad timing gear. If so it can be replaced without removing the engine despite what the books say.

Excuse me. I see that you confirmed the rockers were moving. I’m stumped.

Yes, I so wanted to believe it was one of the two timing gears (It happened on a prior engine.) It would have been a cheap and easy fix! But now that I think about it, on my last head gasket & head replacement two years ago I used that Indian Head stuff, and if I brushed it generously everywhere except the two cylinder bridges (creating two equal weak spots), then add to the equation high-heat, it is conceivable, maybe even likely, that the gasket would blow only on BOTH gasket bridges and nowhere else, resulting in Theory - B to be true.

Again, I cant get to it til tomorrow morning but please keep coming with your tidbits and thanks again.

Any time I run a compression test and see an abnormality, even if it’s a real one, I go back and perform the test a second time with another gauge.

It’s possible to have a problem with the gauge itself or the Schrader valve in the gauge that releases the pressure. Sometimes tiny bits of debris from the cylinder gets blown into the gauge and snags on the Schrader valve. This does not allow it to close completely and when the piston in the cylinder you’re checking returns back down after huffing its way to the top some of the pressure in the gauge will release.

Assuming no gauge problem and universally low readings this would point to a cam out of time or fried piston rings.
By the way, 105 PSI is not normal. A reading like that sucks to be honest. I’m also fully aware that many manuals have garbage like this in print but it is not the truth.

Rule of thumb on compression is 20 X compression ratio at sea level. Knock a bit off for altitude, baro pressure, ambient temperature, etc.

Yeah I think I meant to say MINIMUM 105, not normal. When I had around 30k-mi on the engine I think I was getting 135-145 PSI’s but I digress, because hands down 30psi is nothing. Again its a new tester so no debris, but Im going to have it checked out tomorrow. If debris was lodged in the valve, the pressure needle would drop down and pressure would drop down to nothing and I would have no pressure to release, but thanks for offering that information.

Yes, sorry I didnt mention it but I was in such disbelief on the even pressure the first run that I did check the Schrader valve several times and re-tested compression a second time on all cylinders already.

In addition to the other checkout, tomorrow I will do my “1&4 Leakdown Test” on the “Multiple Broken-Gasket-Bridge Theory,” and yes, I will for the sake of trying everything, stick open the throttlebody during testing.

Anyone have anything else to add on Indian Head Gasket Maker or otherwise, dont hesitate to add more ideas. Thank you so much everyone and talk to you tomorrow!

The ONLY way ALL cylinders can be down like that is CAM TIMING…short and sweet. Oh and the Harbor Freight Comp gauge SHOULD BE HIGHLY SUSPECT… I would def get a different opinion by a normal gauge first…as we might not be talking about such a mysterious issue if we knew what the compression really was. Hmmm

Also check that schrader valve in the bottom of the comp gauges rubber hose the part that screws into the plug hole…sometimes those valves get wonky…

DAMN OK45…I didnt even read your response on the valve…Great Minds…great minds…lol

Autozone has a rent-a-tool program I’ve used in the past. It includes a decent engine compression gauge. I use them so infrequently, I prefer to rent them when I need them.

If the truck were mine I’d squirt a small amount of oil into one of those cylinders and recheck the compression on that one cylinder. If it goes from 30 to a 100 or whatever then you know the rings are gone.
As a formality you could do them all but if one has bad rings then the end result is the same; disassembly of the engine for a rebuild.

JMHO, but 140 PSI on a 30/40k miles engine is not good. Sounds to me like problems were already developing at that point and probably the rings are the culprit.
Severe overheating can seize the rings in their lands and cause a loss of compression.
Other causes for something like this is oil coking or the rings losing their temper (defined as springiness) due to any overheating.

Could you elaborate on this engine failure? What happened? Did it die or suddenly start knocking or what? The reason I ask is because I have a Buick also with this same engine. A few years ago while my parents were using it, the engine suddenly started knocking like crazy. It still ran, though, and the oil pressure light never came on. I dropped the oil pan and removed the timing cover to try to find the noise (by the way, I found a timing chain, not gears. I was surprised too, since I had heard these engines have gears. Mine’s a 1990). My engine had an oil pump gerotor drive system with two counterbalance shafts, driven by a gear on the crankshaft. The bearings for the balance shafts had gone bad. The bad news was that it was a dealer only part and was $350. I did fix it and still drive the car. I mention this as a shot in the dark, but the dealership did say these engines were notorious for those balance shaft bearings going bad.

First, thank you all who offered more detailed insights since my last submission.

Since the Timing Gear Theory was the least-expensive to investigate, I spent a good 3-4 hours just trying to get the bolt off, unsuccessfully I might add (See Attached Photo.) I dont have access to impact tools in the remote/powerless area I have to work on this thing and placing a steel rod in one of the triangulated holes in order to hold the pulley still while turning the bolt doesnt work either. Theres not enough behind the pulley to latch onto no matter what angle the bolt, screwdriver, etc. I try to jam through it. The timing cover almost completely obstructs any metal rods sent through to hold the pulley. Ive called several mechanics but havent located any specialized tools to hold the pulley in place. Most have told me they simply use an impact wrench. Supposedly theres a chain-wrench that latches around the pulley but I dont see how that would work on potentially 200 lbs of Torqued-Bolt. I guess Im just not that strong. Any other suggestions on this issue would be greatly appreciated. My past experience however has been that the path of greatest resistance usually ends up being a wasted effort. So with that Im going back to other troubleshooting.

Tomorrow Im going to pickup my recharged battery and hopefully run the 1&4 leakdown I previously described. I did rent another compression tester and maybe I’ll get different readings. I hope it is just the rings. I know how to swap out the rings on this particular engine without even pulling the whole motor out and it would again be a cheap fix. The theory however appears flawed in that once one or two rings blow the engine fails, leaving minimum compression in the unaffected cyllinders. But I say let it be just the rings, head gasket, or timing gear. Please God I hope its nothing more than one of these cheap fixes!

Additional Background: The previous info should address most replies but I will add that the engine failed in the few minutes idling while I went into pay for gas. The coolant was full at the time and no overheating indicator had been on. Of course no telltale white exhaust or anything like that present either. It did randomly backfire once or twice while attempting to restart but that could be due to various causes.

Again, please view the attached photo/file and tell me if you know of any other non-automated methods/tools available to remove this pulley. Thank you again everyone for your continued participation in this stumper!

With that socket on the bolt and the breaker bar in the socket can the bar be locked against the ground or frame so as that a starter bump cause the bolt to loosen? The theory you are exploring is that valve timing is off far enough to cause this loss of compression? If a look at rotor position reveals it is dead on (with the crank in the right position), you should be able to rule in or rule out a valve timing issue.

I do have a suggestion for removing that crank pulley bolt. I have seen it recommended here before by others and it has received five stars, although the last time I recommended it, the recommendation got one star from someone. It does work, though, so it is worth repeating. Reinstall the belt (or a belt, depending on your accessory layout), take a rag, fold it into a small, dense square, and run it over with the belt by turning the crank pulley with your socket. This will effectively lock the pulley into place so you can loosen the bolt.

Based on your description, the car just stalled and wouldn’t restart while it was parked and idling with no symptoms leading up to it dying. Is this correct? No noises or anything? Does it sound normal when cranking? 30 psi of compression on all cylinders will make it sound unusual during cranking. The engine will spin over very freely. Have you assuredly ruled out an electronic problem and are certain it is a mechanical problem?

Leakdown compression test resulted in skyrocketing compression 165-185psi’s. Had the spark plugs not been full of oil the vehicle probably would have started. Oil was found in the oil filter replaced AFTER cranking during the leakdown test, so the oil pump is working.

I pulled the oil pan and found not a trace of damage:

  1. No other metal fragments or particles at bottom of oil pan;
  2. No scoring on any cylinder walls;
  3. Attempted to jiggle up and down the pistons checking for bearing and ring damage…SOLID!

I dont get it. This is why I didnt want to pull the timing cover; the strong indications were that it was NOT the timing, and I wasted time. Here there are fairly strong indications its NOT the rings/bearings, so I really hate to pull the head and find nothing and have to then replace the head gasket (Im on an extreme budget.)

You’re way off base in your thinking on this. Trying to wiggle the pistons in the cylinder bores will tell you nothing about piston ring condition.

You might clarify this bit about a leakdown test, oil in the cylinders, and skyrocketing compression,
I take this to mean that you did a wet compression test by applying some oil to each cylinder and retesting the compression? If so, this is not a leakdown test.

If this is the case you have a ring problem. Your engine had 105 PSI. You applied oil and now have 165 and up. Rings gone.