The Stovebolt Site Forums The Shop Area Engine Shop Bad Detonation all of a sudden

Hello folks,

I hope you can help me with a problem. I'm a fairly good mechanic, but am at my wits end and seek some advice. First, the truck:
1955 Chevy 1/2 ton NAPCO
Engine is a 1972 small block 400, with 80's HEI distributor and small Rochester Quadrajet, rest is stock including cam

The engine has about 22,000 miles on it, and was rebuilt in 1990. Yes, it does not get a lot of miles on it annually.

About a month ago, it developed a horrible pinging/detonation at anything above the slightest throttle. Here is what I have done so far, in this order:
- Checked timing (was set at 6deg BTDC), and timing would advance to about 34 deg BTDC at about 2200 rpm
- Checked the distributor, esp the mechanical advance. Looked OK, was not siezed, I cleaned and lubed it
- Dialed the timing back to 0 deg, no change in pinging so set it back to 6 deg
- Checked the vacuum adance, no vacuum leak
- Checked the power brake line, no vacuum leak
- Checked around intake manifold for vacuum leaks
- Replaced the distributor cap and rotor
- Took carb to local fuel systems shop, they rebuilt the carb and bench flow tested it
- Emptied the fuel tank bone dry, put fresh 91 octane in

None of the above has made any difference at all. Still pings like crazy, making the truck unuseable. I keep thinking there is something stupidly obvious that I'm missing, but I sure can't think of it. My next step is to replace the HEI and plug wires. Not sure how the HEI could suddenly "fail", but still fire the engine and cause pinging. Then again I don't have much experience with HEI other than this distributor. And maybe it's cross firing across the plug wires? They're 30 years old.

What do you think? Any ideas?

thanks,
John

Read this: https://www.enginelabs.com/news/detonation-what-causes-it-and-how-to-control-it-using-efi/

Try this- - - - -rev the engine to around 2,00-2,500 RPM and hold the speed steady- - - -maybe set the fast idle cam of the carb on the second step or so, and monitor the intake manifold vacuum with a gauge attached to full vacuum (not the distributor). If the vacuum doesn't hold a steady 20 inches of mercury or more, or the vacuum drops steadily the longer the speed is held constant, look for an exhaust restriction. I've seen collapsed exhaust pipes, rodent nets in mufflers, etc. that would cause extreme back pressure and detonation anytime the engine got loaded at all. A quick check is to loosen a pipe at an exhaust manifold and do a test drive.
Jerry

I take it that 34 degrees at 2200 Rpm is with the vacuum disconnected to the advance. That is coming on too soon for a stock cam. About 35 degrees would be about right. Check the advance springs. Check the vacuum advance chamber with a vacuum pump. It should start about 6-10 degrees & be all in at 15-16. It could have a broken spring. Try driving with the vacuum disconnected to the vacuum advance.
Does it do it cold or just when the engine gets hot? Does it sound like all the cyls. knocking or just 1 or 2?

George

I want to hide and watch (behind a very sturdy blast fence) the next time a small block 400 engine turns 22 thousand RPM!
LOL!
Jerry

Yes, about 34 degrees above 2200 rpm, with vacuum hose disconnected. I did check the vacuum advance with a My-T-Vac, it is not leaking and does advance the timing, but I did not measure how much. I did drive it with vacuum line removed and port on carb blocked, it made no difference. It will ping hot or cold, i.e. just minutes after starting it up. And from the amount of noise it makes, I'm thinking it's pinging on all 8. I mean, it is really loud if I put my foot into the throttle much. I had already looked at the advance mechanism. It was not gunked up, just a wee bit rusty and weights and cam moved freely. I cleaned them up and put some lube on them. The springs look fine, both feel the same rate.

At this point the only thing that I've not changed is the distributor. I bought one of those small cap HEI knock offs, and just ordered an external coil. Will give that a try and see, but after that, there is not much left to replace except the whole engine!

-John

Over 30-something years of dyno runs on round track engines, I've discovered that most small block V8's (Chevy, Ford, and MOPAR) develop their best torque at full throttle at around 36-37 degrees of centrifugal advance. Depending on the cam you're running, centrifugal advance should probably max out somewhere just below 2500. If you're detonating under moderate load at low RPM, check the exhaust!
Jerry

Did this come on all at once?
Was there a slow build up to the pinging?
How much exhaust is coming out of the tail pipe? Is it really hot?
Have you tried pouring a cup of water slowly down the carb with engine at fast idle? This steam cleans the carbon out of the chambers.
I have never heard or seen a HEI module act up, they work or they don't.
Have you verified true top dead center and verified the timing marks are correct?
Have you pulled all eight plugs to see how they compare to each other?
Does the engine temp stay steady? or does it climb really fast?
How does the exhaust sound? normal or quite?

I would lean towards plugged exhaust.

Possibly a tank of bad fuel?

Jon.

Some Diesel fuel in the gas will cause exactly the symptoms you're describing, but the exhaust gas will smell like it's coming from a turbo shop heater. The odor is really obnoxious! Also check the timing mark on the balancer against true TDC of the #1 piston- - - - -balancer rings have been known to slip on the hub.
Jerry

Check how much slop is in the timing chain. Ive never seen one jump ahead but I guess its possible with the plastic cam sprocket. If the exhaust is plugged you will hear hissing noise at the tailpipe. If you get a rumbling noise it is probably OK.
George

I'm with Joe H and HRL as far as exhaust restriction. But Joe hit on the timing marks. The harmonic balancer may have slipped. I would time it by ear and/or vacuum and then check the timing marks to see what's what.

SimS

The detonation came on pretty much all of a sudden. About a month ago perhaps.

I have already tried the "hillbilly tune up" by trickling about a pint of water down the carb while running. It didn't puff any black smoke, and did not make any difference in the pinging.

I have not yet pulled all the plugs and compared, that's a good suggesting to look for a possible clue.

I have also not compared actual TDC to the timing mark, another good suggestion. Not exactly the easiest thing to do in this truck, but will give it a go. What I will say is that when the TDC mark on the damper is at ZERO, the rotor in the distributor is still pretty much right under #1 cylinder post. I did check that.

Could it be plugged exhaust? Perhaps. The exhaust note had not changed, still rumbles. This truck has dual 2" exhaust pipes, no cross over, and some really large "big truck" mufflers. Can't imagine what would plug that up, unless some kid jammed a potato in there. I guess I can run a sewer snake up the exhaust and see what I find. Also will hold my hand over each outlet to see if they feel the same.

Just for sake of education, how exactly does one make sure the exhaust is not restricted? I know cat converters do that in time, but this truck does not have them. Do you disconnect the head pipe and hook up the outlet of a shop vac and see what comes out at the end? Never have had to do something like that before.

The engine does not get hot at all while driving it around. It gets up to temperature and pretty much just stays there. Keep in mind also, I can't put my foot on the gas at all other than to maintain a steady speed. Try to coax much acceleration from it, and it is a pinging monster.

As mentioned before, I already drained the tank bone dry and put 7 gals of fresh premium gas in. No difference, still pings. Unbelievable.

The battle continues....

thanks,
John

I wanted to follow up with what I found.

I pulled all of the spark plugs to inspect, they all looked the same and typical. I used a plastic soda straw and found TDC on #1, the timing mark on the damper was basically right at zero deg mark.

Since all of the plugs were out, I figure what the heck, might as well do a compression check. This is where the story goes sour. All of the cylinders popped about 170psi, except for #4 and #6. Both popped 60psi. In fact, with the compression gauge on #4, I could hear and see it pushing back out the carb while spinng the motor over, and with it on #6 a wierd sound coming from somewhere, the exhaust I think.

I have no idea how this translates to wicked bad pinging, but it matters not. Either the head gasket is cracked between 4 & 6, or the head is cracked.

So now I'm on the hunt for a 60s/70s straight six to drop in. The SB 400 was a nice motor, but 8mpg was not particulary a virtue. A 292 six would be nice, but I hear the are fuel thirsty also. So perhaps a 230/250 is in this old truck's future.

-John

It sounds like you have an intake valve stuck (or leaking) on #4 and an exhaust valve leaking or stuck on #6. Take the valve cover and off and watch the valves to see if they're all working properly. You may not have as bad a problem as you think. A valve job might fix the problem.

You've got a blown or badly leaking head gasket between 4 and 6. You're getting enough gas/flame transfer between cylinders that one cylinder or the other is lighting the mixture off too soon, before the spark plug fires. I check for a restricted exhaust system by simply loosening the pipe flange at the exhaust manifold and doing a test drive while watching intake vacuum on an uphill pull. An engine that won't pull the hill easily with the exhaust connected will fly up the hill with a leaky exhaust, and have a lot more manifold vacuum.
Jerry

Deleted my post

Mike B

Indeed I have choices here. Fix the 400, or buy a drop in crate 350, or.... do what I really want to do, and find a straight six to rebuild. It just seems to me these old trucks were ment to be powered by a six, and frankly I do not need the power of the 400 V8. This old truck is my Home Depot, garden center, and landfill runner. It never sees past 70mph as is, and even a 230 six can manage that. Not to mention, this whole fiasco has made a major point of how working on a V-8 in one of these old trucks, not to mention a NAPCO 4WD, is a real pain in the a$$ for an old guy like me. I'm darn near as old as the truck! Straight six is far easier to deal with, and if I'm going to keep this old beast and continue to work on it myself, it's kind of now or never. Think I'll part out this 400, it's a 4 bolt main block, and hopefully pay a bit towards a replacement 250 or 292 six.

-John

If I am not mistaken, all '50s Napco trucks were to have inline 6 engines in them. Anyone that I have known who put a V8 in one, regretted it. Too much horsepower for the transfer case and u-joints.

Well, try and tell that to some one in their 20s! LOL. Here I am 40 years later, and thinking that power is not the be all and end all of the world. Reliability and economy count for a lot as well. Its funny how wisedom comes with age.

-John

John;

Glad to see you found the problem. If I were you, I would pull the head and check the head gasket and valves. It's probably a blown gasket between 4 and 6. Also to R&R a head is much easier than swapping out the motor. The vehicle is only driven 700-800 miles a year so the effort of a swap to bring about the economy of a L6 is kind of a hard to support. Of course if you are looking for a cool project, go for it! With the low mileage, how is the cooling system? Maybe corrosive conditions contributed to the gasket failure.
Good luck.

SimS

Oh, by the way, the transfer case is a Spicer 23, which is a left side drive version of a Spicer 24, same as that used behind big block Fords. The transmission is a Muncie SM465 truck four speed. Both trans and transfer case are beefy as hell and have no problem with the torque of a SB 400. The weak point is the axles. They were not meant to take that kind of torque, especially in the lower gears. Like a fool, many years ago I loaned this truck to a friend who needed it to pull a trailer. He got about two blocks away, and came walking back to my house, and said the truck "just stopped". He thought something happened to the clutch. I walked over and had a look, you could put it in gear, let the clutch out, and nothing. I looked under the truck with it running and in gear, the driveshafts were turning! I locked the front hubs and used that to turn the truck around and head back but got about a block from home when the left rear wheel fell off, with half of an axle on it! Although he swore up and down it "just stopped", my guess is he put it in 1st or 2nd, rev'd the engine and tried to do a burnout with it. (he was that kind of guy)

-John

OK, all fixed. Indeed the head gasket was "gone" between cyls 4 and 6. Have to say, fixing this has been one of the most un-fun things I've done in a while. Between the truck being tall, and the V8 located down deep in the frame, purt near killed me working on it. And worst part was getting the old head gasket off the head and block surfaces. It was like baked on linoleum. The head was bad enough sitting on the garage floor, but the block was a nightmare. Ended up using a hammer and wood chisel to chip it off. Probably would have been less work and far more comfortable to just pull the front clip off so to access the motor. Many, many hours start to finish, but I fired it up yesterday, running better now than I recall for a long time.

One other thing I found, it was mentioned that a plugged up exhaust could cause detonation. Well, that exhaust heat riser valve on the right side was frozen. Not completely in the shut position, but enough to cause restriction. So I guess it's been exhausting through the passage in the intake manifold over to the left side. Not sure if that is what cause the pinging or not, but perhaps the pinging is what cause the head gasket to blow. Will never know for sure, but I replace the riser valve with a "dummy" version with no valve in it. Here in California we don't really need this, it don't get cold like back east.

So now that the truck is running, will be on the lookout for a 292 six to replace this 400 with. There is one up in Sacramento area I may go have a look at. But at least I can take my time about it, have a running truck. Yay!

-John

Nice work. Thanks for posting the results.