Important question about my 37 Pontiac OPINIONS

[Guest bofusmosby]

I dropped my car off at a garage over the weekend, and I finally got word on the findings. First, I was told that the carb. is dumping a lot of fuel inside, and it needs to be removed and rebuilt. I had figured this all along. To give some background on this car, the previous owner was supposed to have rebuilt the engine about 2000 miles ago, and according to him, the rings were replaced, along with the bearings, the timing, a few of the valves (and a valve job) and the oil pump. There might have been other things done, but this is what I remember. Also, keep in mind that I believe that all this was done while the engine was still in the car. This would explain why the engine was never really cleaned or repainted.

OK, with this info given to you, here is the problem and the question I need to ask. The mechanic thinks that this will take car of the majority of the problems, and also take care of the vacuum leak, but there still could be a major(?) problem. When I dropped the car off, I also took the mechanic the actual service manual for the car. He did a compression test, and he said that according to the book, there should be 140 pounds of compression on each cylinder. He said that ALL cylinders are checking low, at about 90 pounds. Does this present a problem? I would assume that having the cylinders un-balanced with some being high, and others being low, this would cause more of a problem. But all of them checked at about 90 pounds. He (the mechanic) said that he might have to adjust the valves, and that this might help, but he wants to rebuild the carb first to see if the engine will run good like that.

What do you guys think? I'm sure that many of you have been down this same road before, so I'd like to hear from you. Honestly, I didn't think that my car was supposed to have 140 pounds on each cylinder, but since I left the service manual with the car, I have no way of checking this.

Thank you guys for any and all opinions on this!

[Grimy]

My Rule of Thumb (ROT) for determining adequate compression pressure for pre-war cars (and post-war flatheads) is to multiply ambient atmospheric pressure (14.7 psi at sea level) by compression ratio (in your case, 6.2), which gives a product of 90.14 psi! Tampa is about as close to sea level as you can get! You're right on the mark!

Perhaps your shop manual shows (as some did) 140 psi as desirable not at cranking speed but at 1,000 rpm, which would require running the engine at a fast idle with one plug out, one plug at a time.

My view is that 90 psi is plenty for your car, and I believe that as long as the cylinders are relatively even in their pressure readings (~10-12% variance), you have nothing to worry about.

Nothing to worry about other than crankcase dilution from that carb! Suggest you rebuild the carb then immediately change the oil as soon as the carb's operation is verified and adjusted.

[1957Birdman]

I think Grimy has nailed it. Just the fact that all the cylinders are the same PSI indicates that everything is as it should be. As long as it runs well after the carburetor is fixed just drive it and enjoy it.

[Guest]

[quote name=bofusmosby;

OK' date=' with this info given to you, here is the problem and the question I need to ask. The mechanic thinks that this will take car of the majority of the problems, and also take care of the vacuum leak, but there still could be a major(?) problem. When I dropped the car off, I also took the mechanic the actual service manual for the car. He did a compression test, and he said that according to the book, there should be 140 pounds of compression on each cylinder. He said that ALL cylinders are checking low, at about 90 pounds. Does this present a problem? I would assume that having the cylinders un-balanced with some being high, and others being low, this would cause more of a problem. But all of them checked at about 90 pounds. He (the mechanic) said that he might have to adjust the valves, and that this might help, but he wants to rebuild the carb first to see if the engine will run good like that.

Thank you guys for any and all opinions on this!

_______________________________________________________________

The first thing anyone working on a car engine in a professional capacity must determine is; Is this engine tuneable? One of the very first things we do is a compression test, and later if we have a problem with that ( low even uniform readings, one low cylinder, two low cylinder together, etc.) we use devises like a leak down tester. We always check the compression as follows;

engine must be warmed up, all spark plugs removed, Carb. Throttle plate blocked slightly open. With the compression tester in the first hole we crank the engine for at least four compression strokes. We note the first as well as the final stroke readings and we repeat this with all the cylinders. reading should not differ more than ten psi. On a average engine of your cars era the compression gauge should show about 75psi on the first turn of the engine if pistons,rings, valves. A few more turns of the engine will give the total pressure reading. THIS IS STANDARD PROCEDURE. Your service manual's compression specification is given in accordance to the above procedures performed. If you have low uniform readings, the next step is to put about a teaspoon of motor oil into the cylinder and crank the engine a few time to get the oil around the rings and then install the compression gauge and repeat the compression test procedure again. If this procedure brings up the compression readings on all cylinders this way, then it is safe to say you have a cylinder to piston ring problem. You can also confirm this by doing a leak down test and this will measure the pressure loss percentage. The leak down test is done on a cylinder that is at top dead center/compression stroke. Air is forced into the spark plug hole and the percentage of leaking air is measured, this air has three ways it leak. 1. intake valve (you can head the leak through the carburetor) 2. exhaust valve (you can hear the leak at the exhaust pipe) 3. Past the piston and rings ( you can hear it through the crankcase breather).

I retired after forty plus years as a mechanic and also working in automotive engineering---The above is standard procedure for your car.

FYI, a VW air cooled engine has about the same compression ratio as your Pontiac, when one of those engines has about 90 psi and even on all cylinders it is generally time for new pistons and cylinder barrels, and when it's generally that low the bottom end is usually at or near the end of it's service life too.

When I'm rebuilding a engine the above case is usual for just about any engine, just like the 428 Pontiac I'm doing right now.

D.

Edited March 29, 2012 by helfen (see edit history)

[Amphicar BUYER]

Even compression is the most important goal. If one was 70 and the rest 90, I'd be concerned, but with all reading equally, it indicates all cylinders are in similar condition and working properly.

Fix the carb, change your oil (and again with filter in 500-1000miles to ensure all crap is out) then add a can of "MOA" (NAPA has it) place your butt in the seat and just drive it!

[Guest]

Even compression is the most important goal. If one was 70 and the rest 90, I'd be concerned, but with all reading equally, it indicates all cylinders are in similar condition and working properly.

Fix the carb, change your oil (and again with filter in 500-1000miles to ensure all crap is out) then add a can of "MOA" (NAPA has it) place your butt in the seat and just drive it!

_________________________________________________________________

So the book say's 140 psi and your reading 90 and it's OK??? Everything working properly??? The people who wrote the manual and built the car know what is best and have have given you the facts. The readings tell me this engine is getting tired EQUALLY on all cylinders.

FYI, this engine does not come with a filter from the factory, but the pan must come down periodically (check the service manual AGAIN) to clean the filtering screen assy. which is unique to Pontiac sixes and eights of 1933-1954.

D.

[Grimy]

D., please read again my post #2 re some service manuals specifying compression readings at 1,000 rpm rather than at cranking speed. I don't have a 1937 Pontiac shop manual, but my MoToR manual covering the subject 1937 Pontiac 6 shows exactly that-- 140 psi **at 1,000 rpm**. Compression readings at cranking speed will be significantly lower, in my >50 yrs experience. I stand by my post #2.

[poci1957]

Just off the top of my head I think for a 1930s flathead an even 90 lbs is probably OK--I would agree to rebuild the carb, see how she runs and call it good. Change the oil afterward like Amphicar Buyer says.

I do agree with Helfen that even but low readings could indicate a worn engine and I agree that the next step is a leakdown test and oil test as he suggests. I have never heard of doing a compression test one cylinder at a time at 1000 rpm--Helfen's procedure is the way I have always done it. I bet there is no problem, good luck, Todd C

[Guest bofusmosby]

Thank you guys for your answers. I not only left the original service manual with the mechanic, but the Motor's manual as well. Maybe that is where he read it. I thought that 140 PSI for that old car was a bit much at a cranking speed, but since I do not have the book in front of me, I have no idea what it says, or at what RPMs this measurement is supposed to be made. I know that the car burns no oil, so I doubt the rings would be the problem. This car ran like a top just a little over a year ago and that was only about 30 miles ago. Very quiet, no smoke, nothing, so I am in hopes that the engine isn't 'tired' and in need of a rebuild.

I'll keep you posted on what I find out. It's going to be several weeks before the carb place can locate a rebuild kit.

[jpage]

Jim ,According to the "Motor's Manual" your compression ratio is 6.2 and the tune up chart lists compression at 105 PSI. at cranking speed. At 90,if all are equal you should be good! Maybe ring gap is a little wide allowing it to bleed off a little.

[Guest 36chev]

Yep, that should be about right. The newly rebuilt engine in the 36 Chevrolet is between 95-100 lbs. per cylinder as I recall.

[Guest]

Thank you guys for your answers. I not only left the original service manual with the mechanic, but the Motor's manual as well. Maybe that is where he read it. I thought that 140 PSI for that old car was a bit much at a cranking speed, but since I do not have the book in front of me, I have no idea what it says, or at what RPMs this measurement is supposed to be made. I know that the car burns no oil, so I doubt the rings would be the problem. This car ran like a top just a little over a year ago and that was only about 30 miles ago. Very quiet, no smoke, nothing, so I am in hopes that the engine isn't 'tired' and in need of a rebuild.

I'll keep you posted on what I find out. It's going to be several weeks before the carb place can locate a rebuild kit.

________________________________________________________________

Jim, you said this engine burn no oil. Every engine new or old consumes oil, it just depends on how much. I have seen engines where oil control was not a problem, however the compression rings were shot. Lets take a look at the spark plugs so they can give us another indication of A/F mixture and oil control.

If you drive this car only occasionally it's possible the engine would last the entire time you own it, however it will not be giving you the best possible starting, performance and fuel economy. If this car is going to be used for touring you might want to consider fixing it properly. Also when you do the recommended oil pan removal to clean your oil pump pick up screen you might want to pull a few bearing caps to see the condition of the bearings and the journals, you will also be able to see some of the cylinder wall to check for scoring. The great thing about these engines is they are a full pressure lubrication unlike some engines of that era like chevrolet which uses dippers on the rods.

A final word on the compression numbers your getting. Look in your manual for the for the lowest acceptable reading. You have said that the mechanic said the book said 140 psi. and now others have chimed in a much lower number, so I think we need to get the facts straight before going any further. If we are basing a opinion on the fact that we have a 140 psi specification and you only have 70 psi, that is a BIG deal. If we are going on a number of 105psi, then 70psi is not nearly as bad. There is no point in going down this road any further until we know the facts.

D.

Edited March 30, 2012 by helfen (see edit history)

[Guest bofusmosby]

helfen

You have no idea as to how much I appreciate your answers, even if it is not something I really want to hear. I trust your experience in this matter, but in reality, I'll know for sure tomorrow morning when I talk to the mechanic. If the engine was being run at approx. 1000 RPM's, then yes, you are correct. BUT, if the mechanic did the compression test while cranking the engine, then 90 PSI may be close to normal for this car. I remembered that I have a copy of the Pontiac Service Manual on a computer disc, and sure enough, it says that the compression should be between 140 & 143 PSI when the engine is running at 1000 RPM's. If the 90 PSI was measured when the car was just being cranked, then I am sure you will agree that the cylinders will not develop the the reading of 140 PSI just by cranking, when it specifies that the engine should be running at 1000 RPMs for this test. By this being your profession for over 40 years, you are assuming that the mechanic checked it according to the book. Believe me, I am hoping that he didn't.

As far as oil consumption/burning, I know that all cars use oil. I was simply stating that this car has never blown any smoke, so I don't believe that the oil is being burned in an excessive amount. Please remember, I am a complete novice at auto mechanics. Everything I do is a learning experience, so please bare with me on this. I am in hopes that in time with your help, along with help from others, I will someday learn.

jpage and 36 chev

Thank you for this info. I am trying to keep calm and not freak out about what might be a major problem. As I just said to helfen, after I speak to the mechanic tomorrow, I'll know for sure. I'll keep all of you posted as to how the mechanic did the compression test. For the time being, I'll just have to keep my fingers (and toes) crossed.

[Guest]

Jim, just one final note on compression testing. The reason most books say that you only need to let the engine turn over four times through the compression cycle is that after that and for however many more times the pressure generally remains the same. Now to be honest with you I have never heard of checking compression with the engine running-but if the Pontiac service manual says this is the way it's done, then that is the way to do it. A note on Pontiac engines of this era. When these engines are idling they are running so slow that they are almost at cranking speed of some cars. My 455 Pontiac actually cranks at nearly 500 rpm, and guess what? my dad's old 1950 Pontiac straight eight hydra-matic idle specification was 350 rpm in drive! So testing this way is plausible but very time consuming. Still, if compression is supposed to be 140psi and it's 90psi engine performance in all aspects is compromised.

D.

[Guest bofusmosby]

Perhaps the engine is not up to par, but wouldn't you agree that the normal compression on the cylinders would be greater @ 1000 RPMs, compared with say 200 RPMs? With the 6 volt battery, the engine doesn't crank fast at all. Yes, it cranks steady, but not fast like a 12 volt car as you well know. 1000 RPMs with my car would be a fast idle, not a slow one. Of course, I have never tested to actually see what the RPMs are, but according to the service manual, the most horsepower with my car is about 3500 RPMs.

If come to find out , the engine is in need of a rebuild, then this will be a game changer for me. I'll have to cut my losses and sell the car. I do hope that this isn't the case.

[Steve_Mack_CT]

Jim, before you get too concerned about the car and decide to cut your losses, how does it run overall?? So there is question as to whether the engine has appropriate compression which should be figured out. BUt does it go down the road ok? Do you have a frame of reference i.e. a simillar car to compare that to? Does it skip, run rough or have issues starting?

Agree with the compression concerns, although I tend to think you are ok based on my experience with engines of that era. A pro rebuild on the carb is probably a very good idea, they are simple, but if you are uncomfortable with it, then getting it done is probably not a bad place to spend a few dollars. As Don says a big clue will be if the engine is "tunable" - after the carb I would suggest cleaning or replacing the plugs, and frsehening or setting points - essentially do a tune up. If the vehicle is not smoking now as you suggest, has adequate power and runs well, you should be good to go. After all, you are not commuting daily in this thing, right? Bear in mind cars of this era are not "gas and go" and many small issues can arise regularly.

My suggestion given your climate and the wonderful gas we have these days is to have the mechanic set the float up to give you a slightly richer mix than spec - and possibly route the gas lines as far from the exhaust as possible while he is in there anyway. My guess is your biggest issue with this vehicle will be related to poor gas/vapor lock, but again, a description of what you are experiencing will help as well.

Nice car, bought at a good price, that you seem to enjoy - take a breath before talking about sending it down the road - many times a simple fix is all that is in order!

Edited March 30, 2012 by Steve_Mack_CT (see edit history)

[Guest bofusmosby]

Thank you Steve. I have the car in the shop right now, and even though the car was running badly, I (and the mechanic) are in hopes that it will run just fine after the carb is rebuilt. They are taking care of that issue. There is a place in town that does nothing but rebuild carbs, so I will be calling my mechanic this morning to give him a heads up about where to locate a kit for the rebuild. California Pontiac Restoration sells them for $36, so I'll be calling them in a bit to make sure that their kit has everything needed, not just the gaskets.

I'll post the answer my mechanic give me about how he checked the compression. Believe me, I have no intention of selling my car, but if the engine needs a complete rebuild, then I am afraid that if I were to keep the car, it would be sitting for many years before this would be done. This may force me to sell the car.

[Steve_Mack_CT]

Good luck with it! "Running badly" is pretty general, BTW - making it a little tough to give advice there.. Apologies in advance if I missed a post where you detail it - but symptoms are as critical as tests to resolving the issue...

Another great source for carb related kits, floats (they can be expensive), etc. is Daytona Carb right in your back yard.

Hang in there, it's a long way from a troublesome carburetor, or points being out of adjustment to a full rebuild, with lots of potential steps in between...

[Guest bofusmosby]

Thank you again Steve. I too don't think (from what I was just told) that I have an internal engine problem. I spoke to the mechanic this morning, and he told me that he measured the compression while he was cranking the engine, and not while it was running at 1000 RPMs. He also went on to say that he doubts that there is any kind of internal engine problems, just the carb. I guess time will tell.

BTW, here is a link to the original thread I had started more than a year ago about how the engine was running. This should give you some kind of idea as to what has been going on.

http://forums.aaca.org/f120/37-pontiac-engine-not-running-right-295369.html

[JFranklin]

I think the time for a total engine rebuild is when it won't run any longer, or is putting out a smoke screen, and then it may only need a ring and valve job and bearing care.