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This is my 90 744 with regina ignition and original engine.
I wanted to at least check compression in #3 before ordering any parts and before registering this. It'll be a little extra work and cost with a notary to get it registered, since my late friend didn't list his wife on the title.
Anyway, I warmed the car up, put the gauge in #3, and left the other plugs installed, pulled fuse 1 and cranked the engine. First reading was 30, but I hadn't gotten the gauge seated fully. I tightened it again, and got a whopping 65. I stuck a glove on the oil fill port and saw it pop off when cranking along with some vapor. The PCV was in terrible shape (cracked seam) on this.
So I think "ugh, probably needs a head gasket... dang it." I added 2mL of oil to the cylinder and put a little on the threads and o-ring too. I got 70psi. I decided to check the others. #2 and #1 *both* read 70psi, and I knew there was a leak onto the timing belt because the top waterpump seal is balooned from oil touching it.
I lined up the crank mark and the cam mark is one tooth to the left of aligned with the dot on the timing cover, maybe 1.5 teeth. That would put the cam advanced 1 tooth, I think (judging from the outlet of the water pump, I think these spin CCW). It definitely needs a new belt, as this one is showing cracks in the back side of it, and the cam seal.
To make sure I'm reading the crank timing marks right, it's -1, 0, +5, +10, right?
Is 70psi across the board (ok, I didn't feel like doing #4) reasonable for being 1 tooth off and having left the plugs in? That seems like a big change from the ~175psi I would expect for an NA engine.
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Problem solved!
Stinkin compression gauge did not work. It maxed out at 60-90 or 100psi. The new one ran up to 160 for all 4! I even tried them both at the same time in different cylinders.
I also had an issue where the wire from the alternator to the intake manifold had a 0.5V drop from alternator to the manifold. I added another one and got the battery charge up to (according to my cheap battery tested) 80% rather than 30% for the test.
All tests but the last one done with the throttle closed. The last one was the same as the others :P
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This post has been marked as an answer to the original question.
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HI,
Well, I guess I learned something too!
The next time I read up on past posts I will try to remember to ask this question.
Do you hear the starter laboring itself through the compression strokes while cranking?
70 verus 160 psi is an appreciatable difference. At least you appreciate it! (:-)
The next time you might try holding down the pressure release while it strokes and observe the reading.
If you get nothing at all then be suspicious of something as not being right.
This is because not all of the volume of a reasonably large cylinder should be able to escape through that tiny side port.
The needle has to raise about half way and change upward even more if the release is not used on the next stroke.
It would be like switching from a really bad cylinder and then over to a good one!
What you also need to do, before throwing away that gauge, is to let it do one more thing for you!
Open it up and see the simplicity of how they are made.
Observe how delicate the mechanism is. It measure the flex of a flatten curl of a copper or brass tube.
All it takes is a good drop or a thump from the right direction to disrupt the fragile gear train inside.
Imagine the first pocket watches and how much redesign work they had to go through to get a survivor. Many were not cheap to make, but they got that name anyway, when it was how they were treated!
I will say everyone pays a price to become more educated! I opened one of them myself! (-:) Let alone countless other things? (:-(. Like Art's well documented necropsies provide us!
I'm Glad you got the car straighten out and so willing to tell on yourself too!
You are a credit to being a standup fellow.
Thanks,
Phil
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OK.... take 2 is not much better. But why? All plugs out this time and a spare battery pack hooked up to keep the battery happy.
I adjusted the cam timing back and forth and it always seemed 1.5 or .5 teeth off. The belt is worn, so maybe that would account for half a tooth...
Anyway, several tries later, it idled like poo (obvious misfire) every way I tried it.
I ignored the timing marks on the harmonic balancer (HB) and went by the actual top of the piston stroke just looking through the #1 spark plug hole. Based on that, the HB is definitely slipping because TDC is at about -25 degrees.
BUUUuuut - I set the cam timing based on the top of the piston stroke and am still getting only about 90-95psi. Adding 3mL of oil was enough to make oil droplets float around my garage (oops) and the air coming out the spark plug holes was forceful enough to blow the paper towels out of the ports. It did not change the readings at all, so it doesn't seem like bad rings to me.
I suspected the gauge was reading low, but I managed to get it to read 120psi with my portable 12v tire inflator (the inflator read about 140psi). That was using a piece of a nitrile glove as a gasket and mushing the inflator onto the end of the gauge hose. Maybe I'll rent another one tomorrow, but I think these values are going to be the same.
The cylinders are FILTHY. Like I could scrape a layer of gunk off of them. I would expect that to make the readings higher than normal. I'm hoping maybe some of the reason for low values is valve deposits that could get cleaned off.
I haven't started it up with the top of stroke timing yet, but I wonder how it will run like that.
The thing is that if the compression is actually that bad, I don't want to spend money on it and I don't want to sell it to anyone who isn't going to work on it themself. I'm getting to be short on time and would much rather finish and drive my 89 wagon that I think is only waiting on me to assemble things.
Can someone help me brainstorm here?
EDIT: I just realized maybe the valve shims all have negative clearance and are being held open.
EDIT2: I just put a borescope in cylinders 2 and 3 and they both have valves showing some funky junk on the seating surfaces - whether that's just hydrocarbons I can clean off or rust, I think that cranking the engine like I have been is getting worse compression than I would get from the engine running. Also I recently got a battery tester and I attached it. It showed only 38% current state of charge, so the engine was probably turning slower than ideal. It's on the charger now. Finally, I'm 100% sure that the balancer is off because I can see the notch in the washer on the crank timing pulley and it is aligned with the mark on the front main seal carrier.
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I wouldn't recommend ever taking compression readings with any spark plugs left in. The engine must be allowed to spin freely.
I also wouldn't have thought that such a small (apparent?) error in cam timing would affect cylinder compression. Advancing or retarding the cam timing slightly is regularly done to improve performance.
I think the fact that all cylinders you have tested are very similar is a positive sign. Get a different compression tester and recheck all cylinders with all plugs removed, and report back please!
Ian F
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Agreed.
The most reliable results are obtained when the engine has been warmed up, spark plugs and air filter removed and giving it full throttle while cranking the engine several revolutions.
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Agreed that the truest readings are from a warmed up engine with a fully-charged battery and plugs out. I didn't bother with the plugs out because I just wanted a pass/fail for the one.
The throttle and air filter steps are unnecessary. The WOT is a throwback to some EFI systems where cranking at WOT shuts off fuel injection, but it won't affect the values. I've tested this on a dozen engines (throttle open vs closed) to prove this point to a mechanic who swore it would dramatically change the outcome. There is plenty of air inside the intake manifold and combustion chamber to push it into the small volume of the hose and pressure gauge.
Once I fix the timing, I'll get a true reading - plugs out.
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Hi
I agree with you statements about air flow into the engine during a compression check.
That is why I didn’t beat you up on having the throttle wide open.
If you are trying to start an injected motor they don’t want up to use the throttle at all!
Beside compression is compression to get a motor starting but it’s turning speed is very relevant along with voltages to the ignition system during a cranking!
How did the option of blowing air check out in any cylinder check out?
I diagnosed a 454 cid engine once that had one cylinder with low or almost zero when cranking.
Turned out the intake valve was over tighten at the factory!
It was being held open, ever so slightly but eventually got so bad (missing) to become very noticeable by my step father. A couple Chevy dealers, in his home state, had shined him on!
When I backed off the valve rocker arm and started the engine, the cylinder started knocking like hell! He thought I had hurt his engine! Oh boy?
I then explained to him that the piston ran too cold, for too long!
At higher piston speeds it was firing, but not so completely.
It should be a warranty issue! He calmed down!
Luckily GM put a newly remanufactured engine in for free! It had a 50,000 mile warranty.
I told them want I had found but I saw that the engine was written up under some other failure code for a Pistons something or other.
So, GM never knew the truth!
It this explains, to me, why the mid Eighties were so bad for them!
I got the Motorhome at 43,000 and have installed heat shields, a bigger radiator and a rebuilt transmission since into his gift to me. It now has 90,000 miles but it’s doing great, guzzling gas, but it MOVES that nearly 10,000 lbs!
I’ll keep hoping it’s just a cam timing thing for you too!
Phil
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Hey, Phil :)
No dice on the compressed air yet. I work 13-hour shifts 4 days this week, and don't have a compressor here, unfortunately.
I tried to do a leakdown test on the 94 b230ft I'm rebuilding on #3, and could hear the air escaping from the crankcase and oil dipstick tube. It's a good idea, I just couldn't try it yet.
I looked at the cam again and realized the engine rotates *clockwise* so the cam is retarded a tooth or two, not advanced. The cam has 38 teeth, so each tooth is like 9.8 degrees.
That's a heck of a story! I wonder if the ring lands got damaged from the constant stress of having one valve closed and still going through the motions. Seems hard on a set of ring lands to be pulling on what should be a burning mixture during a power stroke with a dead miss. Was that carbed or fuel injected?
I just fixed up an 86 Chevy 366 high-deck for my late sister's work truck. It used to be a Snap-on truck. After rejetting the Holley carb, fixing an exhaust leak, and putting in the all-mechanical distributor and setting high-altitude advance, it has a lot of giddyup - except when it won't downshift on a long somewhat steep hill. It feels good having a beast moving after doing some serious work on it.
Fingers crossed it is just the cam. I'll try to adjust the time this Friday.
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Hi,
The engine is with a Rochester carburetor and it’s in a P30 chassis. It’s a heavy TRUCK with a Holliday Rambler aluminum upper structure.
The engine ran up to about 25,000 miles, I think, before he had me look into it. I inherited at 43,000.
If it had been the exhaust valve held open it wouldn’t have lasted so long. It also would have been more noticeable from the exhaust note or a weak suck back patter.
I don’t know anything about “ring land” wear but you don’t want to put those in upside down.
Incomplete combustion will dilute lubrication on the cylinder walls.
When we took it to a dealer in my small town and I left it running. The mechanic walked half way to the vehicle and said oh! That’s a bad engine all right!
You’ ll have to take it to the next dealer 20 miles away to get that replaced under warranty as they can do major work up there! I immediately got forgiven! (:)
I doubt that shop even opened the engine up. Just shipped in a Chevrolet remanufactured engine.
It says so, right on the valve covers!
Yep, if the timing marks are off any at all, you will have to reset it!
Depending on the condition of the belt how deep you will have to dive into it right away, if the belt looks fresh.
It’s possible that the adjuster was not maintained often enough and the belt could still work for a little while, to get a synopsis of what is going on.
A lot of people don’t realize that an engine rocks backwards, slightly, when shutting down.
The Pistons will equalize against a compression stroke with the lack of momentum..
The chances are pretty high that this has probably happened.
I like adjusting the tensioner after every other oil change. So that’s about 15,000 miles.
If people go in for “quick lube” shops, nothing happens to it!
It only takes a couple minutes to loosen and tighten back one nut under the rubber plug!
Somewhat faster than a messy oil “strainer” exchange.
I change the belt @ 60,000 miles, fairly faithfully, but I did lose one after all these years, just a few years ago. I have six cars and I’m pretty sure I messed up, as I have aged liked my cars.
Check you later!
Phil
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Ahh, yeah! The truck I've been working on is an 86 P60 (p6t42). It's a BIG box truck compared to a volvo. I swear I could spend thousand bucks in parts to get it up to snuff for me if I were going to be driving it.
The ring lands are just the spots on the piston that the rings fit into. I was talking to the machinist when deciding to get my b230ft block bored out to the first oversize about it. As the piston travels up and down the bore, the piston rings expand and contract, especially when there is wear in the bore. The more wear is usually in the middle, and so the rings expand and contract a little on each stroke. That eventually wears the ring lands, which can fail.
They can also fail for a few other reasons like ... I think if the rings are installed with too little clearance between the two ends of the same ring. Then when the engine warms up, the ring loses the gap and goes from a C to an O shape... then if it keeps expanding, that circle that should be flat (when viewed from the side) starts to bow like a split washer and can break the ring lands.
I posted an update up at the top ^^^^. We're all aging, btw. One thing I learned is to forgive myself for being human. Humans make lots of mistakes!
Cheers!
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Remove all the spark plugs and hold.the throttle @ Full Open when doing the compression test . Ground the coil wire.
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Needing to hold the throttle open is a myth that won't die.
Definitely will remove the plugs for an accurate test. I started out just wanting to check that #3 was not showing 0psi.
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Hi,
What’s very unusual is the fact that all the cylinders are low!
Engines shouldn’t wear out or fail that evenly, IMO.
The idea that the cam is way off with the Pistons strokes would affect all the cylinders.
Removing all the plugs will allow the engine to spin faster as the momentum of the crank would be equal and therefore, will be more accurate.
Take compressed air from a blow gun with a rubber tip and blow it into each cylinder.
Have each one parked on TDC or with the valve cover off, get the cam lobes pointing skyward over each cylinder.
If it doesn’t move the engine around or air comes up into the intake manifold or out exhaust manifold then you really have a serious problem with the valves not being closed.
I hope this will give you an optional method way of troubleshooting.
Phil
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Trying to determine TDC on a 740 engine by using the damper pulley marks can sometimes be questionable. Chances are you have a two piece pulley with a rubber isolator ring between the inner hub piece and the outer pulley ring. With old age comes the possibility that the outer ring has spun off of its original relationship to the inner hub.
Carefully gauging the top of the #1 piston stroke can help you find TDC but that’s not always easy to do. The most reliable way to determine absolute TDC is to remove the damper pulley and lower belt cover to witness the mark on the crank sprocket. You have to do that anyway if you replace the belt.
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Aye, I'm aware these can separate. If they did though, I think my timing would probably appear to be something like 12 teeth off, not just 1.
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So it “appears” to be just 1 tooth off, but it COULD be 12 teeth off if someone tried to time the engine, using the damper marks, AFTER the pulley had already started slipping.
I would put a plastic drinking straw in the #1 spark plug hole and crank the engine over to as close to TDC as you can sense with the straw. It won’t be exact but at least you can see if the timing is anywhere in the ball park. I agree that if your low-but-equal compression readings of 70 psi are really caused by a cam timing error then the timing must be WAY off.
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I owe you a beer... but also the compression reading was still only 90 after I fixed it, so... beer for everyone?
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The method you suggest to verify TDC - with a straw or chopstick - is a great deal more accurate than ball park. One tooth off is impossible using your method.
--
Art Benstein near Baltimore
“Never underestimate the difficulty of changing false beliefs by facts.” --- Henry Rosovsky
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