400 motor questions - Pontiac GTO Forum
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post #1 of 31 (permalink) Old 07-13-2016, 09:55 AM Thread Starter
 
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400 motor questions

Well, finally got the car back from the mechanic. Ended up replacing the starter, radiator (bought a 4 core radiator from Ledfoot Racing), and did some rewiring. Car is running pretty strong. Compression is level across all cylinders (between 110-115), except cylinder 8 (showing 88). Hoping driving it for a little might fix it a little (he said the rings look ok).

My question...I took it out for a drive the other night. Temp gauge never crossed the halfway mark, even idling in traffic, and it was 92 degrees out and pretty humid. Brought the car home, pulled into the garage, and it died. Wouldn't crank, wouldn't even buzz when I put the key in the ignition. I let it sit for a few hrs, then tightened down the battery connections. However, the motor was still hot to the touch, and that was after sitting with the hood open for 4+ hrs. I'm going to attempt to start it this afternoon. What would cause the block/heads to still be that hot after that time?

Car is a numbers matching 68 GTO, 400/4 speed, 51k original miles.

68 Canary Yellow GTO
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post #2 of 31 (permalink) Old 07-13-2016, 11:48 AM
 
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Seems odd to retain heat that long.

Your compression is only 110-115 PSI? That's awfully low for a '68. I would expect to see at least 165-170 with no more than a 10% difference between the high and low. How was the compression check done? Who performed it?

Let us know if it fires up when you give it a shot this afternoon.

When the only tool in your toolbox is a hammer, you tend to see every problem as a nail.
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post #3 of 31 (permalink) Old 07-13-2016, 12:34 PM Thread Starter
 
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He ran the compression test when he was going over the motor. I'm not a big motor guy, I know the basics and that's pretty much it. I'll go over the work list he did when I get home and look at the compression numbers (I hope I remembered them correctly).

What could be the possible causes of the engine holding heat for that long? The car sat in the garage for over 20 years, so he pulled the heads, made sure the crank spun, did all the loose ends, etc. I'm hoping it's not a coolant issue.
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post #4 of 31 (permalink) Old 07-13-2016, 01:02 PM
 
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For the record, the correct way to do a compression check is to first pull all of the spark plugs, then block the carburetor wide open (the engine has to be able to pull in ample air to compress), then check each cylinder with four "puffs" only. If it was not done in this manner you will not have an accurate test.

Earlier, you said the temp "never crossed the halfway mark". Do you have an actual number to associate with that? I'm not overly familiar with the factory markings and calibration, as I have an aftermarket numbered gauge in mine. Did you verify a full radiator with adequate flow? Depending on ambient temp, it may have just taken a long time to get all that cast iron cooled back down.

As for not starting, make sure you're in neutral and that your neutral safety switch is working properly. Then, go from there.

Good luck,

Chuck

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Last edited by chuckha62; 07-13-2016 at 03:02 PM.
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post #5 of 31 (permalink) Old 07-13-2016, 04:23 PM Thread Starter
 
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Well, it started and ran with no problems, but even after a short drive, I can't believe how hot the motor was. I'll have to look for the compression numbers when I get back home tonight. The temp gauge has no numbers, but the needle stayed on the "cool" side of the halfway mark the entire time.

As for the radiator, it was filled, but I don't know the extent of the flow. I noticed some slight screeching of the belts at higher rpms, but the tach currently isn't functioning, so I can't tell where it is in the rev range. I'll need to replace the belts sometime in the near future.

What would cause cooling issues? The fan is still the stock unit that came with the car (from what I can tell). I need to find out what motor I have in the car (400 w/ a 4 speed).
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post #6 of 31 (permalink) Old 07-13-2016, 11:51 PM
 
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You've got a lot of possibilities with a "new" car, but check your timing - I had exhaust manifolds turn red in a Cavalier that must have been blowing out flaming fuel.

Does fan have a clutch? If so, see if it's loose - not sure how to tell if clutch locks when it's running, but if it's slipping it won't pull air through radiator at low speeds. I assume you have a fan shroud. Make sure the belts are tight enough.

Was radiator water rusty? Any sign of coolant leaks on old radiator, like water marks on radiator core? someone could have put in stop leak and clogged up small coolant holes in heads.

Does it have original heads? See casting number above center exhaust ports. This relates to compression ratio. And if compression is stock, someone could have backed off timing so it doesn't detonate. Might also want to make sure distributor is original and that weights are free, springs doing their job, etc.

A handy tool is non-contact infrared thermometer. Harbor Freight has a cheap one $20. Accuracy may be questionable, but you can tell how much it's cooling down.
Infrared Thermometer - Non-contact, Digital Thermometer
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post #7 of 31 (permalink) Old 07-14-2016, 09:31 AM Thread Starter
 
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Thank you for the tips. I haven't really had time to inspect or tinker around with anything yet. My mechanic has been working on my cars for 20 years, and has rebuilt several 60s and 70s model Vettes, so I have faith that he didn't screw me over.

As far as the clutch fan, I'll check it this weekend, as well as the belts. They were replaced 5+ years ago, but the car sat for such a long time.

As far as the radiator, it had no leaks when we brought it to the shop and got it running. When he took it for an actual drive, he got back to the shop and it was leaking, so I just went ahead and ordered the new one.

The heads are original from what I can tell (I was told the motor was never touched), but I'll double check the casting numbers (again, I need to check the block to find out what 400 I have). From what I was told, the distributor is fine.

Again, I'm not a big engine guy. I know the basics, but none of the nuances of Pontiac motors. This will all be a learning experience for me. Thanks again for the help guys!
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post #8 of 31 (permalink) Old 07-14-2016, 12:39 PM
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If the heads are original, you should have about 180PSI compression in all cylinders. Was the timing chain messed with? I suspect it could have been installed a tooth off, resulting in retarded cam timing and low compression. Also, you need to get a pyrometer and verify your engine temps compared to the gauge...I suspect the gauge could be inaccurate due to a bad sending unit or gauge. It almost sounds like the engine got hot and seized after you shut it down. Can you turn it over with a breaker bar on the crank pulley nut? Lots of things to start checking here. Also, I have found that MANY compression testers are inaccurate due to faulty schrader valves. Be sure the compression tester is in good shape, or verify compression with another gauge. NAPA sells the new schrader valves for about $2 apiece.
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post #9 of 31 (permalink) Old 07-14-2016, 12:57 PM Thread Starter
 
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The engine started and ran with no problems yesterday, it was just heating up quickly. When it died the other night, I suspected the battery terminals. After tightening them down and letting it sit overnight, it started on the second try. I'm thinking I'm gonna have to chase down a bunch of little things to get this solved.
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post #10 of 31 (permalink) Old 07-14-2016, 04:27 PM
 
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Yeah, a bunch of little things... I agree with all that's been suggested. I've been working on them for 45 years but have much to learn about Pontiac specifics from guys here.

First have to establish temperature. Sounds like the gauge shows okay, but I don't trust them. You can get a cheap mechanical temp gauge from Autozone ($20) to verify it. Coil up the lead and tie wrap it somewhere under the hood. Pontiacs run kind of hot and there's a lot of metal to cool down so is important to get numbers (accuracy not as important as relative.. how fast it goes up and how long to cool down). If it's REALLY hot, it'll boil over. I read a crude test is spit on the head and if it spits back it's too hot...

It could have stuck thermostat (remove it for testing). And there's a plate with some tubes from water pump to block - either can rust through and cause circulation problems.

You've got one low cylinder (#8 ) - hopefully your memory was wrong about the other 7... Could have a head gasket leaking into water jacket (not sure if this causes overheating). Can tell by pulling radiator cap (when cold) and putting plastic wrap loosely over fill inlet, push it down to make a dimple, then secure with rubberband to form air seal. If you start it (or just crank it over a few revs with coil wire pulled) and the plastic pops up, you've got pressure in water jacket from cylinder. It doesn't take long for the water to rise from expansion(?) with running engine as it heats up so don't let it run long.

Good to pull #8 plug to see what it looks like (less carbon than others can indicate water leak). Could be carboned/stuck rings causing low psi if not head gasket, so could spray something in hole, let it set, see if it loosens, and check comp later (others have more experience with this - not sure if good to spray PB blaster, WD-40 as it may be bad/wash cyl walls if you start it - Marvel Mystery/regular oil may be ok).
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