TAC Tech => Mechanical => Topic started by: red79phoenix on November 21, 2021, 03:53:11 PM
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Hello, still trying to track down why my 1979 403 trans am has died on the highway 10 years ago. - Here is the old thread http://transamcountry.com/community/index.php?topic=83545.msg2414#msg2414
TLDR
- Backfired out carburetor and died when downshifting coming off the highway
- Short Block rebuilt <5000 miles ago.
- Compression is low in 6 of the 8 cylinders (<50 psi).
- Confirmed valvetrain is moving with valve covers off.
- Last recommendation in above thread was to pull off timing cover and see if it jumped a tooth.
I took the timing cover off and there appears to be a lot of wear on the sprocket teeth - the chain is cutting grooves in it. - correct me if this is not unusual.
I see the dimple in the cam sprocket - how do I check correct placement of the chain? IIRC I align the dimples in the two sprockets?
Here are some photos - https://photobucket.com/u/adam_collard/a/c7ea5a82-3d0a-4cb0-a29d-bd0953122be8
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I would be very concerned about the wear I see on the lower gear on the crankshaft. Seems like a lot for only 5000 miles. It also looks like metal fragments on the crank gear. There should not be any slop in the timing chain.
Proper cam timing should look like this photo. Mark on the cam gear will be in the 6 o'clock position and the crank gear will be in the 12 o'clock position. The marks are often just like a center punch dot, but could be different like a triangle point. Or like what is shown on the upper gear in the photo. You will need to remove the oil slinger to see the mark on the lower gear.
(https://live.staticflickr.com/65535/51695252012_efa1a6a097_k.jpg) (https://flic.kr/p/2mL8Fou)IMG_2795 (https://flic.kr/p/2mL8Fou) by Lawrence Alexander (https://www.flickr.com/photos/188552644@N06/), on Flickr
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in the photos,
there are some plugs behind the cam gear, one of which should have a small hole to lubricate the timing chain
looks mighty dry to me, any chance they are not installed correctly or blocked
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in the photos,
there are some plugs behind the cam gear, one of which should have a small hole to lubricate the timing chain
looks mighty dry to me, any chance they are not installed correctly or blocked
It sure appears that there is an oil problem... I have been cranking the engine probably 10-15 minutes without getting it to start which may explain the wear. There is some oil pooled in the fuel pump drive.
I added a photo to the above album link. I rotated the sprocket and the dimple appears to line up with the crank dimple like 5th T/A's photo. So, unfortunately the timing has not jumped :(, I was hoping that explained the problems.
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As said, the wear on the crank gear is not normal and not good. I agree it looks like an oiling problem. The odds that metal got into places it should not be (like crank/rod bearing journals) are high.
You said the timing gears and chain are intact and the dots align per the posted pic. One minor comment - on PONTIAC engines, the number one cylinder is in firing position when BOTH the cam gear dot and crank gear dots are at the top (12 o'clock). When the cam gear is at the bottom (6 o'clock) and the crank gear is at the top (12 o'clock) the engine is in position to fire cylinder number 6.
I DON'T KNOW for sure if it's that way or not on a 403. The 403 "may" be in firing position #1 when the cam gear is at the bottom (6 o'clock) and the crank gear is at the top (12 o'clock)... older Chevrolet engines follow this line up.
Did you check compression in all 8 cylinders? Is it only 50-ish in all 8? Can you see the cam lobes? If the cam was totally fried (due to improper lubrication) that would explain the low compression
Sounds like you may have bigger issues, but it seems weird it isn't trying to fire, even if it's backfiring etc. Did you verify that the distributer rotor turns when you crank the engine? The distributer gear may be damaged. Are you getting spark? Fuel?
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Also.... you did not say for sure, but the timing dots have to line up with cylinder #1 at top dead center on the compression stroke. Just lining up at some point doesn't tell you anything.
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As said, the wear on the crank gear is not normal and not good. I agree it looks like an oiling problem. The odds that metal got into places it should not be (like crank/rod bearing journals) are high.
You said the timing gears and chain are intact and the dots align per the posted pic. One minor comment - on PONTIAC engines, the number one cylinder is in firing position when BOTH the cam gear dot and crank gear dots are at the top (12 o'clock). When the cam gear is at the bottom (6 o'clock) and the crank gear is at the top (12 o'clock) the engine is in position to fire cylinder number 6.
I DON'T KNOW for sure if it's that way or not on a 403. The 403 "may" be in firing position #1 when the cam gear is at the bottom (6 o'clock) and the crank gear is at the top (12 o'clock)... older Chevrolet engines follow this line up.
Did you check compression in all 8 cylinders? Is it only 50-ish in all 8? Can you see the cam lobes? If the cam was totally fried (due to improper lubrication) that would explain the low compression
Sounds like you may have bigger issues, but it seems weird it isn't trying to fire, even if it's backfiring etc. Did you verify that the distributer rotor turns when you crank the engine? The distributer gear may be damaged. Are you getting spark? Fuel?
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Yeah, I checked all 8 cylinders compression - 55 100 85 90 35 50 40 30 were the measurements.
Yes, I have fuel and spark, and yes the distributor is turning.
The car tries to start - gets really close and then dies. I'll try to look at the cam lobes and double check the number 1 cylinder is at tdc. When the car first died I did drain the oil and did not find a bunch of metal shavings - I think the wear on the sprocket might be new.
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So before I add in my 2 cents, just to be clear you haven't touched the motor in 10 years? You are just now getting back to it to figure out what went wrong?
If so that opens up a multitude of other issues besides the one that caused the problem you are trying to solve.
If there is any kind of rust or gunk that has built up over the years of sitting, that could be causing low compression. Same thing with the cylinder walls having surface rust possibly not getting a good seal with the piston rings.
Since you have the motor apart now, I'd check the basics like have been suggested above. Additionally I would be checking for things like a bad coil, bad ground and confirming that you have fresh gas with a good known carb. Might also be a good idea to spray down anything in the motor that could be metal to metal at this point. Pull off the valley cover and pour some oil in places it hasn't been in years.
Keep us posted!