Smoking after rebuild


Hi. I rebuilt my small block chevy and installed it back into the car. It ran fine for 200 miles. No issues. But the other day it suddenly started smoking real bad. And I mean A LOT of smoke. It does NOT smoke when cold. It only smokes after having been idling for a few minutes. Reving or fast idling the engine does not change the volume of smoke. No noises or other issues with the car (a '77 Camaro). In fact, it starts and runs great except for this.

Here’s what I’ve tried so far:

!. Disconnected the trans vacuum modulator line. No change.

  1. Disconnected the PCV and oil breather. No change.

  2. Tested engine compression. All cylinders are 125-140. All sparks plugs except #3 had fresh oil on them.

  3. Tested cylinder leakdown. All cylinders 7%-14%

  4. Checked for vacuum at the dipstick tube (to check for leaking intake). No vacuum. Intake gasket presumed good.

  5. Replaced valve stem seals. This was something I knew had to be done because I had installed the wrong type during the rebuild. Most of the seals I replaced were in various stages of disinigration. Unfortunately, this did not solve my smoking issue (although it might have reduced the smoke a little)

So that’s where I’m at. I’m about to pull and disassemble the engine but I want to explore all other possibilities first. Much of what I’ve read on the internet suggests it could be a break-in issue, that it needs to be driven more. Could that be the issue even though it did not smoke for the first 200 miles of the break-in period? Any other ideas?

Thanks. Curt.


When you first started the rebuilt engine, did you allow the engine to idle at 2,000 RPM’s for at least 20 minutes before driving the vehicle?



I hate to be the Grim Reaper here, but those compression numbers suck to be honest and oil on the plugs means that there is more than likely a piston ring and/or cylinder wall issue.

If you go back and perform that compression test again with the wet method (meaning a squirt of oil in each cylinder before it’s tested) you will probably see those 125-140 numbers go up a lot.
That means rings unfortunately.


I wonder if you’ve got the correct crosshatch pattern on the cylinder walls . . .


I agree with ok4450, the leakdown numbers are pretty poor. The rings did not seat. Do you have non detergent oil in there? It helps seat the rings. OR Did you maybe use standard rings with a 0.030 overbore?


Tester - yes, the engine was idled at high speed for about a half hour. No smoke upon startup.

ok4450 - I’ll redo the compression test wet and post the results. 125-140 is bad? I thought it was normal.

db4690 - the machine shop did put a crosshatch pattern in there. Couldn’t tell you if it was correct or not.

Mustangman - I don’t remember if the oil was detergent or not. The pistons & rings were correctly matched to the .030 overbore. That much I know, since I mic’d everything upon assembly.

Thanks for the comments. Keep 'em coming. I’m gathering though, that the consensus here is that it is bottom end trouble.


A radical cam would explain the low compression #'s, but not the high leakdown.
Did you clean the heck out of the cylinders with hot soapy water and rags after honing?
Did you check valve stem to guide clearance? If the guides are worn the seals won’t make up for it.


You’ve mentioned some detail on the diagnosis but what about the rebuild? Did you ensure the rings were right side up? Did you clock the gaps? Did you use assembly lube to install the pistons into the bores or did you do them dry? The heads were apparently not gone over prior to re-installation- was the engine running soundly before the rebuild? Any reason to suspect warping or cracking in the heads?

One concern I would have is this scenario- the valve seals and guides are trashed. They are installed onto the fresh short block. The copious oil leakage from the heads causes the rings not to properly seat during break in and even though you have now corrected the leakage, the rings are not properly seated…


bore cylinders. hone cylinders. new pistons. new rings. all rings installed up/down correctly? heads have new valve guides and new valves. right?


Before you fitted the rings to the pistons, did you place the rings in their bores and check them for the correct end-gap? Also, how many different parts make up the “oil ring”…Those expanders that support the two scraper rings, they like to get out of place, the ends overlapping instead of making a butt-joint…I preferred ring-sets that used a one-piece oil ring… Good luck, hope you don’t have to tear it down again… What flavor small-block is this? Is it stock?


Thanks for the responses.

circuitsmith - Mild cam. Cylinders were cleaned prior to assembly. Valve guide clearance was checked with a dial gauge and within spec. You consider those leakdown #'s high?

Twin Turbo - I installed the rings myself. It was my first time but believe I did it correctly. Correct side up. By “clocking” the gaps if you mean offsetting them then yes. Assembly lube was used. The block and heads were hot tanked, magnafluxed, and checked for straightness by a proffessional machine shop. Valve guides are original, but as I mentioned within spec. The engine had not been run for 15 years prior to the rebuild.

Cavell - Cylinders were bored .030 over. Cylinder wear was actually quite minimial for a 30 year old engine. The only reason I had them bored is because I got a little too aggressive with the ridge reamer when tearing it down. Valves, springs, rods, & rockers are all original. The only valve components that failed inspection and were replaced were the lifters.

Caddyman - Yes, I tested fitted the rings prior to installation and all gaps were within spec. That oil ring setup is certainly fussy. It’s possible I messed up on a couple, but I don’t think 7 out of 8. It’s a 305. All stock except for a mild cam and the overbore.

This was my first rebuild, so it’s certainly possible I messed something up. I measured everything 3 times just to make sure I got consistent results. The block & heads were proffessionally serviced. I used a standard rebuild kit from a major supplier. I knew I messed up on the valve seals. I also think I messed up the rear main seal because I have some leakage back there. Beyond that, I thought the rebuild process went quite well and am still scratching my head over this.


Those compression numbers are very low for a non-racing cam. When I did the head gasket on my 1994 F150, I got 160 or so readings out of a engine with 170K mi on it, with factory 8.8:1 compression. I would expect a freshly-rebuilt gasser with mild cams to be approaching 190 PSI on a compression test…unless it was a 70’s smog-choked engine with deliberately low compression to combat NOx or something.


Back in 1979 I bought a 1977 Camaro RS with the 305 with about 16k miles on it and from my hazy memory I seem to recall compression numbers on it were about 180 or so at around the 40k miles mark.

It had a few quirks but was generally reliable and underpowered. Luckily when I put it up for sale someone paid more than what I owed on it so I banked a few grand anyway.


Did you check the ring gaps when you did the rebuild?


Are you sure the block was bored and honed and not just bored?


If you’re confident about the ring prep/installation and cylinder bore job, I’d be more inclined to investigate from the top side down before I pulled it all the way out again.

First, despite the prior PCV test, I would check the manifold for the presence of oil.

Next, I’d pop the valve covers and pull the hardware off a couple of valves at a time to check the backsides of the valves for fresh oil. The stem seals could be wrong again (sorry!) or failing in some way and it’s a heck of a lot easier to check than the other stuff…

BTW- the smoke is bluish tint versus bright white, right? Just trying to make sure you don’t have two separate issues…


Meanjoe - mine is as you described… a low compression smog gadget-choked engine from the late 70’s. But as ok4450 pointed out, my compression is still low. The car does seem underpowered to me, but I always assumed it was just a small block trying to haul around 3500 lbs.

mountainbike - yes, ring gaps were checked.

oldtimer - When I dropped off the block & heads I asked about honing. The owner of the shop said that honing was automatically included with the boring.

TwinTurbo - for being my first time, I’m fairly confident I did the rings correctly (at least there were no “Oh #%&!” moments). Valve covers have been off as well as all the rockers when I changed the seals. But the only part of the valve I could inspect was the stem so I’m not sure I understand.

A possibly related clue is that the engine runs rich. And after doing some more internet research I’m going to pursue the possibility of fuel diluting the oil (and thus allowing it past the rings), as well as some of the suggestions posted here that I haven’t tried yet. I’m also going to change the oil & plugs just to start fresh. Unfortunately, some house issues have come up so work on the car is going to slow down for a couple days. But I’m still at it and will post some follow-up test results. Thanks again.


OK, finally got a chance to do some more work. Here’s an update.

Changed oil & filter. This is the first oil change since the rebuild. The old oil was thick and black, but otherwise free of debris or metal shavings. Cut open the oil filter and all I found was a little gritty sludge in the paper. So the engine is not disinigrating.

Cleaned & regapped the plugs. Same condition as previously noted (most had wet oil on the threads and a dry black coating on the electrode). I’m thinking of going to a hotter plug.

Retorqued the intake manifold.

Leaned out the carburetor. I know it’s running rich and I didn’t want any rich condition black/gray smoke obfuscating the diagnosis.

Started her up and waited for it to come up to temp (remember, it never smokes when cold). While waiting I noticed that #7 plug wire was arcing to ground right at the boot. Upon inspection I saw that there was a small tear in the boot, probably from taking the wires on & off a hundred times. Checked out the rest of the wires and 4 of them had similar damage to the boot and wire #4 was also visibly arcing to ground. So I made a temp repair with silicone and electrical tape. New wires on the shopping list. BTW, those wires ARE new. They’re expensive Bosch wires and I’m a little disasspointed in how quickly they failed.

Started it back up and checked the timing. Still at 8 degrees where I initially set it.

Took a vac reading. When the engine was cold the Vac was steady at 16. When it warmed up the needle floated between 15 & 16. That’s low. Does it make a difference that I’m at 5,000 ft. elevation?

Once it warmed up I got the blue smoke again. But it’s not nearly as bad as it was on the 1st day. It’s not bellowing smoke now. It’s lightly puffing it out. But it’s still there and it’s definately blue.

I’m going to put a couple hundred more highway miles on it to see if that changes anything. After which I’ll re-do the compression test. And of course pick up a new set of wires.


OK, took it for a nice long highway drive and we definitely have a pattern here.

  • At high speeds (60+) there is NO smoke, whether steady or accelerating.

  • At medium speeds (40-50) if I decelerate I’ll get a little smoke out the back.

  • At medium or low speeds, if I decelerate and then immediately accelerate I get a huge cloud of blue smoke. Even a very slight modulation of the gas pedal will produce a cloud of smoke.

  • Whether I’m coasting, accelerating, or decelerating, the smoke is worse the lower the speed, down about 10 MPH.

  • Regardless of what speed I’m at, or how bad it’s smoking, I can almost immediately clear up the smoke by flooring it.

So with this additional information…rings or guides?


I will suggest that you pull the intake and inspect the valve stems for oil. But the weak compression and excessive leak down are troubling even if valve stem seals were the cause of the smoke. Maybe you should pull the engine and go back through it.