rocketraider

Buddy's 77 403 LeSabre wagon is in a snit like I have never encountered. I've done everything I know to do.

Back in late summer it developed an occasional EXHAUST backfire on deceleration. Eventually this busted up the converter element and it plugged. Poor car barely made it home from Fall Carlisle.

Since backfire generally indicates ignition issues: we have tuned it, new plugs, cap/rotor/coil, and plug wire resistance checked great so we left those alone. The old plugs were clean and if anything appeared to have been running lean so I don't suspect fuel carryover into the exhaust. Timing is dead on, vacuum and mechanical advance are working. EGR is disabled. No vacuum leaks we can find and the carb's power piston and metering rods are free. Choke working correctly and secondary side functions. I rebuilt this carb back in March, set everything to spec and everything had been fine till the backfiring started. It gets fed non-ethanol fuel almost exclusively unless it needs fuel and ethanol is all that's available- but it does it on both fuels.

After all this and it still had no power, he took it to muffler shop and found the converter plugged. Remedied that (won't say how) and this car still backfired deep in the exhaust- sometimes sounding like a gunshot. As I feared it would do it blew the back end of the muffler open. Replaced that and before he made it four miles home it had backfired and split the muffler again.

I don't know what else to do. Any ideas? All I can think is unburned fuel is somehow getting into the hot exhaust and lighting off but I do not know how. Throttle plates DO close off and I epoxied the well pugs and put the carb kit's foam plug in the throttle body.

Koda

My guess is hairline crack in ignition wiring that engine de-torqueing on decel causes it to flex and lose electrical contact for a bit, effecting a miss or two, engine leans back over, power, on, ka-boom.

1970cs

My ex-wife had a Plymouth that would backfire on deceleration. We had a shop look at it and they replaced the distributor (worn bushing) it ran well after that.

Pat

76olds

Getting good spark at each plug ?

Eric

matt69olds

Greg Rogers

Does this car have a belt driven air pump? I once had a 73 Nova with a air pump and I accidentally hooked up the vacuum hoses wrong so it didn't divert the air on deceleration. When you let off the gas in a coast the air continued to go into the exhaust and made a huge back fire boom. Hope this helps, usually if you have a big enough bang to blow up a muffler it almost has to be air (oxygen) is entering the exhaust system causing the rich unburned fuel in the exhaust to ignite. Good luck, keep us posted.

Greg Rogers

Could egr valve be bad, even though it is disconnected, Allowing air to come in exhaust? I dunno, just throwing out ideas....

tru-blue 442

It does sound like an ignition problem. How many miles? (timing gears). A cracked sparked plug or a bad wire as
mentioned above? When you did the tune-up, did you replace the ingnition module in the distributor?
That would be my best guess. Good luck and let us know.

Charlie Jones

Why is the EGR disconnected ?
This engine was tuned and jetted with the EGR operation in mind .
I would try connecting it up properly and make sure it is working .
That may just solve your problem .

olds 307 and 403

Do the rotor movement test, it could be a worn timing chain that skipped a tooth. As said replace the plug wires as a precaution. Gutting that cat should have restored the power. Could tell balancer have slipped and the timing is off?

droldsmorland

Start simple..Ignition. Open the hood in the dark with engine running. Look for arcs.A bad rotor is hard to see this way. This is one of those times I say try a high quality replacement.
How are the engine grounds? HEIs dont like bad engine grounds. The ignition module could be failing do to this or its age.
Inspect the 12 volt feed wire to the distributor as well as the coil terminal both at the coil and inside the plastic connector. The wire could be broken inside the casing ahead of the terminal. Hard to see this one, but have diagnosed this before.
How does the centrifugal advance look? Can you see it actuating/"coming in" with a set back timing light? If the advance is too aggressive at low RPMs it could cause the farting, especially combined with a worn timing chain or cam.
A bad EGR usually causes a stumble not a back fire.
Look for an exhaust leak up close to the engine.
Rule out all possible vacuum leaks.Is the vacuum signal steady at idle? If no thats a bad cam or a vac leak(lines, PCV and brake booster lines etc).
Look at a leaking intake/exhaust valve(leak down wet/dry compression test)
No mention of mileage on this engine? Higher miles Id look at the timing gears/chain as mentioned and dont rule out a bad cam lobe. Generally a bad cam lobe will pop out the carb thou.

70W-32

Do a compression test to check for burnt exhaust valve or bad exhaust cam lobe. Replace ignition wires with quality set, make sure cap and rotor have no internal arcing

matt69olds

Greg Rogers

This seems like a good possibility also, I would think if it was plugs, wires, cap, etc you would have a misfire on acceleration not coasting...

Charlie Jones

I think you may have nailed it Matt .
As I look back at my career as a mechanic I remember changing a number of " pole pieces " with broken or almost broken wires .
They either didn't run at all , or they exhibited "intermittent ignition" , backfiring , blown mufflers etc .

rocketraider

I had wondered about timing chain with 140k miles but the timing is steady at idle and checks good with dial-back light. The EGR has been disabled as long as he's owned the car and running without it has never been an issue. My Cruiser dislikes EGR intensely, stumbling and spitting its way off idle.

A couple of old-line guys here have suggested either charcoal canister or distributor wiring, so those will be our next steps.

Me Toronado developed a weak wire in the distributor and would shut off without warning, It would always restart immediately so that one was fun to track down.

Thanks for all the suggestions! I told him it was unseemly for a Buick to carry on like that, especially an Olds-powered Buick!

patshotrods

Glenn,

I hate to hear that you and Tim are having such a problem with this, please let me know if I can help!

1970greensupreme

I'm thinking this car has an air pump, the pump should not be pumping air, aka 02 into the exhaust on decel. If it does and you dumping fuel on decel that will cause the un-burnt fuel and 02 to fire in the exhaust. Most have a vac valve and temp valve etc so it will pump air to manfiolds cold, then downstream once warmed up but on decel should dump the air out of the air pump muffler.

rocketraider

77 does not have air pump. 81 307 is the earliest I've seen that on a 49-state Olds V8.

I just hate to start throwing parts at it. There's some niggling reason it's doing this backfire, just have to find it.

tru-blue 442

Any chance of dropping in a known good HEI dist in it? May be worth a shot.

GoodOldsGuyDougie

don't decelerte, Just keep moving like in that movie speed!

it's time to laugh again!

shiftbyear

2x on the timing chain, if Olds still used the nylon coated gears in 1977 I'd be checking for chain slop or jumped teeth as Olds307and403 suggested. Here is a pictured of an original Chevy gear I recently took apart. The nylon is completely gone. Many good suggestions on this post,

Charlie Jones

There's an easy way to check the timing chain .
Rotate the engine with a breaker bar and socket on the balancer bolt . Rotate it in the normal direction until the timing mark reaches zero .
Remove the distributor cap .
Turn the engine backwards very slowly until you just notice the distributor rotor move . ( You may need a helper for this . )
Note the number of degrees on the timing scale , this is the amount of "slack" in the timing chain .

Anything more than 4 degrees is cause for replacement .
The engine will still run at 6 , but poorly .
Anything 8 degrees or more is usually "out to lunch" .

frankr442

I'm with Charlie Jones and post #9. My 85 307 ran just fine with EGR hooked up. It was the only emission part I left on the car.