I have a 1953 pontiac Chieftian, and I have been having trouble getting it running. I did a compression test and the numbers are as follows. (Flat six and a hydromatic trans, has high compression stamped in the head) 60 60 55 70 65 65 once I found these numbers i put some oil in the cylinders and almost got it running than a little more and it finally fired up. The motor seems to run very well and when you rev it, it increases the RPM very crisply, the car has no exhaust instaled and it has next to no smoke coming out of the manifold. Once i shut the motor off i tried to restart and had trouble fireing it up, once again i got it running and let it run for 30 minutes, shut it off, and again it was slow to start but fired up quicker this time. Questions. What should the compression PSI be for this motor? The test i did was with a cold motor, and throtle wide open. This car has been siting for an unknow amount of time, but my friend fired it up last fall. I'm assuming these compression numbers are way low, and would indicate a poorly running motor, but the thing runs like a top, and it smokes less than my infiniti. Any thoughts here? stuck valve that unseized? Thanks for all help, and any input! -Chris
A couple of thoughts about your engine. You never said if adding oil helped the compression readings? My book says 132 psi with a 20% max variation. So doing the math results in something between 106 and 119 psi for the lowest cylinder. The book also says that the engine must be warmed up before doing the tests too. I would retake the compression readings with engine warm and throttle wide open. See what happens.
7.7 compression ratio should give you 100-110 psi. If it has set for a while, a shot of oil in the cylinders will give you a better idea. Comp checks need to be done with a warm engine an wot.
I don't know if the oil helped the compression, but it without a doubt helped it start. Next week when im working on it again, i Will warm up the motor and re-do the compression. 50 sounds a long way off of 132, but it runs so nice. Thanks for all the help.
How many turns did you spin it on each hole? With big chambers it takes a few comp. strokes to get an accurate reading. Fully charged battery is a must, too, especially a 6 volt. If it doesn't spin fast, your readings will be low.
This might help you see how compression testing on Pontiacs was supposed to be done, at least back in the '60s. This procedure should help you some with your old Poncho. Here is what the Dealer Sevice Bulletin number 67-1-27 dated 2-3-67 says: Low compression pressures obtained on GTO, 421 HO, or 428 HO engines are NOT a valid indication of engine condition. Due to the long valve overlap period with camshafts used on these engines, compression pressure readings (at cranking speeds) as low as 120 PSI are considered normal. An engine should NOT be condemned as faulty on the basis of these low compression pressures alone. The readings obtained from all 8 cylinders should be taken and recorded. If the lowest cylinder is not less than 80% of the highest, the engine compression is probably okay. If there is still some doubt, the engine may be tested using a cylinder leak tester. If leakage at TDC on the compression stroke is not over 20%, the cylinder is okay. Compression pressures should be checked as follows: 1. Run engine until completely normalized - 15 minutes minimum 2. Shut off engine and hook up primary tachometer. 3. Remove all spark plugs. 4. Disconnect coil tower lead at coil and ground to engine. 5. Block choke and throttle open fully. 6. Install compression tester in No. 1 cylinder and crank engine at least 5 compression strokes. Observe cranking speed and record highest pressure obtained. 7. Repeat for other cylinders. Be certain cranking speed is the same for each check (within 10 RPM) and is at least 150 RPM. 8. Take 80% of the highest reading. If no other reading is below this figure, the engine is probably okay.
Hi R Pope I did about five turns on eacy cylinder. It is possible my battary was a little low. Hi Draggin GTO, This is interesting food for thought 80% off seems to be a lot, but they built the motor. Is is possible that the compression on the motor is ok, and adding the oil simpley aided the starting process by bumping the compression higher? I'm curious as to why this motor was so hard to start but started very quickly with oil in the cylinders. Thanks again for everyones help! Chris
The oil in the cylinders sealed the ring/chamber creating higher compression, allowing the motor to fire. As noted above, you need about 90 psi to get fire. The motor will continually be hard to start because your compression is too low. I had the same problem and if the car sat up for a long time, the rings are probably rusted against the piston...at least mine were, creating bad compression. Another potential issue is the valve lash. If the valve lash is not set correctly, one of the valve could be still open during the compression stroke, leaking off your compression. You can verify this by doing a leak down test. Use the compression tester hose with an air chuck. REMOVE THE VALVE STEM from the compression gauge hose. Put the hose into the spark plug hole WITH the gauge on it. BY HAND, turn the motor over until the compression gauge begins to register. (This might be difficult to see with very low compression - use your finger first if so). This puts the cylinder in TDC with both valves closes (in theory). Take the gauge off the hose and hook up the air chuck. You will get air out of the tailpipe (exhaust valve open), out of the carb (intake valve open) or in the crankcase (bypassing rings). While in this position, check the valve lash. you should be able to slide a .012 feeler gauge blade between the lifter and valve shaft on both valves. If not, try .004 or something small. If you can't get it between the two, lash is wrong and holding open the valve, losing that cylinder pressure. I know this sounds like a lot, but I just suffered through this on a freshly rebuilt motor. When finally corrected, car starts right up immediately now. It's a 53 Pontiac also, L8 though.
chris, couple things, on the first part, 5 turns does not equal 5 compression strokes. which is what they want. turn that 5 into twenty. basically until the gauge stops going up. as for the 80% difference, that isn't what they are telling you. they are saying a 20% difference. if 100psi is your highest reading, multiply that times 80% that gives you 80 psi in this case (how convenient of me) if your lowest reading is 80psi or above than you are within the 20% that they would call good. those numbers are completely made up, but you get my point.
Thanks for all the above help, I will need to investigate further the next time I'm working on the car. Would poor valve lash, or rings rusted to the pistons not lead to a poor running smokey eninge? This may be wishfull thinking, but is it possible the rings were stuck than became free once the motor warmed up? JDholmes- do the intake and exhaust manifolds need to be removed to adjust the valves? Thanks again for all help! Chris
OK Old engines take old methods !! Put quart of Marvel Mistery Oil in the pan. If you can find something like "Casite or Rislone" pour that slowly down the carb at 1500-2000 rpm. Some Marvel down the carb will work also in a pinch. A word of warning it will be SMOKEY while doing this. LOL The marvel in the pan will work wonders loosening up gummy rings,but will need to run a while ,driving it is better.
It is possible the rings were stuck, but if you are having the same problem with the same solution each time, chances are not (IMO). The manifolds do not have to be removed to adjust the lash. However, I'm 6'4" and getting behind the exhaust flange and into some of those positions is not pleasant. Oh, and the simplest way to check and set is by firing order once you get TDC. I looked more carefully at your compression readings so probably disregard the comment about rings being stuck. I'd have a hard time believing they were all stuck about the same amount.
I recovered an old motor with the same issues, once. It spec'd out at 120 psi, but was mid 60's on 3 outta 4, 90 on that one. This is the one cylinder that had the valves tightly closed while sitting. An old timer told me to fill the holes with oil and turn it over a while, then repeat. After 2 or 3 trys, it had enough compression to start. I dribbled tranny fluid in the carbs with it idling (big white smoke!) for a few minutes, drove it around a bit, repeated. After running it a few days, it came up to spec on all 4 with only a couple of psi variance.
Thanks for all the help. The car sat for a week and fired right up today. Whatever was causing the low compression seems to have fixed it's self, it was probably the result of sitting too long. -Chris
I'm with JohnEvans, a little Marvel Mystery goes a long way in freeing up sticky parts. I'd keep running it , make sure it has good oil pressure, and see what happens.
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