Backfiring 93 Lesabre

[Fuely]

Hope someone can give me some advice on this Buick. Have been working on this car for some time now with no results. 3800 engine has a misfire and if the engine is revved to around 2500-2800 there is a constant popping in the exhaust manifolds. Sometimes it will shoot a flame out the throttle body.I have been through the ignition. Coils/Module/Wires/Plugs Crank sensor/Cam sensor. Have checked fuel pressure and regulator all seems within spec.Even replaced all fuel injectors. Though maybe a valve or valvetrain problem. Had cylinder heads off and did find some leaking exhaust valves and had everything repaired. Still has the same exact problem. No computer codes either. Can any one give me some advice? I have no clue.

[Bassasasin]

Wow.. not sure where or exactly how the overrev controls kick in but they do somwhere.... wonder what the tach would read during that. I think it cuts back fuel pulses.

More serious thoughts are timing chain or dampener loose..

Where that smart Moderator??????

[Fuely]

I have been thinking timing chain for a few days now, so what do I have to lose? Probably have 80 hours in this car already! Thanks.

[spinne1]

I have noticed a pattern with this era cars being that I have two 92s and have completely rebuilt one engine a few months ago because of a very rough idle, and am now rebuilding the other one's engine because of high mileage and because it appears to need it. The pattern is that the camshafts will fail. The entire reason for the poor idle on my first 92 was due to a failed lobe on the camshaft (engine had 172,000 miles on it). Now, this probably was not caused by the camshaft failing itself, but rather because of accumulated sludge causing the valves to stick, causing misfires and cam lobe damage. The second 92's engine I now have completely apart and have discovered that the camshaft is in need of replacement on this vehicle as well (197,000 miles). There is no severe damage on the lobes as there was on the first vehicle (the first vehicle only had one damaged lobe actually), but there is pitting and the beginning of lobe grinding on about 4-5 lobes.

The one common denominator is how filthy the engines were on the inside. This is especially true of the valve areas. It is easy to see how they could stick with so much muck.

Note that the valve stick theory is just that--my best guess. I have no real proof as to what is causing the cams to fail.

You might want to check your cam. A good way to preliminarily test before tearing the engine apart is to perform a compression test (which is how I discovered on my first engine that a tear down would be necessary). On that vehicle with only one bad lobe, but with a horribly rough idle, my compression numbers were 150, 150, 150, 150, 127, 110. The low numbers were for pistons 1 and 2, which are directly across from one another, so I knew it could not be a head gasket issue. I thought it might be piston ring failure because adding a teaspoon of oil to the combustion chambers brought the compression numbers almost up to normal (maybe 135 in the 110, and 145 in the 127).

Because I tore it down, I replaced the piston rings and main and rod bearings, along with the timing chain & gears, dampener, cam sensor magnet, water pump, as well as the one bad lifter (the others looked perfect and therefore I couldn't bring myself to spend the money on such an old car), and lots of other little things like some sensors and vacuum hoses and what not.

I thoroughly cleaned every part in the engine with soap and/or mineral spirits and wire brushes and white shop towels (about three dozen per car), and compressed air to dry out some parts, such as lifters that had filled with soapy water (I later soaked them in fresh oil as well as the push rods). It took about two months to do it for the first car and I'm four weeks in on the second car so far.

Try the compression test and post your results and we'll see what we can figure out from that.

EDIT: note that my rebuilds do not include any boring. I did hone the cylinders but that is it. Why? Because my compression numbers were good (aside from the low numbers caused by cam lobe trouble), therefore the cylinders seemed to be in good shape and I attribute the slightly too low overall compression numbers to worn rings. The engines both have had good power even before the rebuilds and therefore I did not feel it necessary to have any boring done. I did check the pistons with a micrometer to see that they were in spec and they were. I also checked the crankshaft and it was nearly perfect.

Note also that the first 92 I did is now my everyday driver and it runs like a top. Even idle, good power and no trouble at all. I have peace of mind knowing the engine is spotless inside (for a while).

[Loekee75]

My 92's 3800 misfire problem was solved with a timing set/ camshaft sensor replacement (at around 138,000 miles). There was also a very fine crack on one of my brand new spark plugs which caused misfiring as well.

[auto trainy]

When cam lobes fail a valve will not open or not open enough,it just might be an exaust valve not opening enough and when you intake valve opens it pops through the intake manifold.The quickest way to check is to remove the valve covers and with the engine running(messy) look for a rocker arm thats not moving up and down as far as the rest,this will indicate a worn camshaft lobe or possibly a weak or collapsed lifter.Good Luck

[Fuely]

Thanks for all the advice on this problem. I checked the compression and found the following :Cyl 1 195 Cyl 2 190 Cyl 3 195 Cyl 4 195 Cyl 5 192.5 Cyl 6 195 Do you feel that a bad cam may still be the problem? The engine acts like it has a vacuum leak,but I can find nothing. When its idling, the idle will surge up a bit and then return to normal.With a scanner connected,the IAC counts will increase then go back down. This will happen every 30 sec. or so. When you found the cam bad in your engine, did it have the hydraulic roller tappet or just the standard flat tappet cam?

[spinne1]

Originally Posted by Fuely

Thanks for all the advice on this problem. I checked the compression and found the following :Cyl 1 195 Cyl 2 190 Cyl 3 195 Cyl 4 195 Cyl 5 192.5 Cyl 6 195 Do you feel that a bad cam may still be the problem?

No, it does not seem so.

Originally Posted by Fuely

When you found the cam bad in your engine, did it have the hydraulic roller tappet or just the standard flat tappet cam?

Hydraulic roller.

I have no clue what is going on with your car.

[Bassasasin]

Still think its the timing chain. But blowing flames ... sounds 180 out.
Maybe loose enough to allow a blow by at RPM... not at a compression check..
Fuel flow often causes surging....

Good Luck

[HotZ28]

Originally Posted by Fuely

I have been thinking timing chain for a few days now, so what do I have to lose? Probably have 80 hours in this car already! Thanks.

If I had a 3800 engine that was popping and backfiring, the timing set & magnet, would have been one of the first things on the agenda. In fact, when my cam gear magnet disintegrated, the symptoms were very similar. No codes either! Chain was sloppy loose @ 153K. Replaced everything about two years ago and she is still purring like a kitten.

I assume that you have checked the firing order many times by now, if not, here it is, so you to do it again.


Type 11 Delco ignition:



Type 1 Magnovox ignition:



Good Luck!

[Fuely]

I will pull the timing cover Monday and check the timing set. Will let you know how it looks. Thanks.

[HotZ28]

You may want to inspect the magnet first. Pull the cam sensor & rotate the engine until it is visible. (You may need a mirror to see it under the water pump.)
If the cam magnet is not sensed by the cam sensor during rotation, you have one chance in six, that the injectors will fire in time with the valve opening. (More than likely, they will be firing out of time.)

Wouldn’t you really rather have a Buick?

[Fuely]

Well I removed the timing cover and the chain and gears (190,000) and they are extremely worn and sloppy. So I am putting it back together tomorrow.Hopefully this will help. Let you know.

[Fuely]

Well the cars running again and the surging is gone. Still getting the popping noise in the exhaust though. So I will keep looking. Least Im getting somewhere with this beast.

[Bassasasin]

Thanks for posting your results.... It is interesting...

Looking at the popping... Does it idle and pop while idling... or just while running down the road?

When I hear you describe popping, I think popping exhaust is from reburning gas.
Maybe a more explicit discription.. like pops when letting off the throttle or burying it into the firewall..
Im not sure if the catalitic converter is part of that problem.. I wonder if it stinks? Sulphur.

How was that cam magnet?
Does it still flameout the intake?
Can you disconnect the camshaft sensor and it changes the symptoms of popping.. more ... or less... ??

Good Luck.