Backfire thru the throttle body

[BobNSam]

Let us know...us armchair troubleshooters would like to know how wrong/right we could be...hopefully off on the cheap side

Bob

 

[gwcowgill]

Bob. like I used to tell my students, My crystal ball broke years ago so let's get the hands dirty.

 

[phish555]

I thought id try putting a timing light to it before i take it in. Im timing it from the bottom of the engine, (so not #1 cyl) but im not sure which cylinder im suppose to use. I was thinking # 8. Could someone chime in with the correct cylinder number. The indicator im using is on the bottom drivers side. Also, im suppose to disconnect a timing bypass wire and im not sure where its located? Any imput on that is appriciated.
I have thought about what everyone has said about this problem, and perhaps some more information is needed. When its warmed up is when the problem starts. It backfires through he throttle body, BUT, only backfires one time on each occurance. So not multiple backfires. I also experience a loss of power at that point. So far, i replaced the fuel filter, rotor and distributor cap, fuel filler cap, and i have cleaned out the throttle body. Anyway, i thought id try timing it as a last resort before taking it to a shop.
Thank you all!

 

[PCarillon]

you time the motor using Cylinder #1 ... will it gradually accelerate up in rpms if you slowly press gas pedal with out back firing.Back firing under load could be a bad coil

 

[phish555]

yes, cylinder # 1 when looking at the timing indicatior from the top side of the moter. BUT, im using the one from the bottom side of the motor.

 

[BobNSam]

BIF.
4 degrees BTDC with tan/black wire disconnected near dist (not 4 wire).
This should provide base idle timing before computer makes changes to advance point.
This should cause a computer error code which should be cleared after timing checked.
If you can get a hold of a real time code reader to hook up to the ECU, it might help point to problem. Some autoparts stores will loan you one.

 

[phish555]

Ok, trial and error allowed me to discover that it was cylinder #5 that i hook up the timing light to. It was firing at 2 degrees btdc, i moved it to 4. I also talked to the shop where i bought the RV, and they said it sounds like its attempting to vapor lock. Hmm, anyway, i took it for a drive after i timed it, and had one backfire, again after it ws good and hot. but it otherwise was ok. Not sure what to think. Still may have to take it in. Thanks to all who contributed.
Glenn

 

[DiGiacomo]

I guess i can chime in here too..........i live at 5300 ft. .......if i set my timing to what the book says.......i get an engine that won't run right...........i have a timing light and have used it for getting an engine 'in the ballpark' to make it run........then i use my ears..... and drive it to listen for backfire [either intake or exhaust].............i have been lambasted for this practice, but i have set all of my chevys this way........

i guess my advice would be to play with the timing a little more.........since the light made a difference in the performance............also i think someone mentioned to check your vacuum advance..........

 

[phish555]

Ok guys. I took it in for service. Got a call and they said i need a new timing chain for sure, and that they could move the harmonic balancer 1 inch before the rotor would move. I am going to go down there and have them show this to me. BUT, i also had a friend who is very knowledgeable about the 454 and said i could have a worn exhaust cam lobe.
What are your thoughts??
Ill give feed back from the service shop when i get back.
Things i have done so far: New fuel filter, new fuel filler cap, new rotor, coil, and distributer cap, and i have timed it.

 

[BobNSam]

Got a call and they said i need a new timing chain for sure, and that they could move the harmonic balancer 1 inch before the rotor would move. I am going to go down there and have them show this to me. BUT, i also had a friend who is very knowledgeable about the 454 and said i could have a worn exhaust cam lobe.

Moving the harmonic balancer one inch before getting the rotor to move can be an indication that the timing belt is stretched although you can only be sure of this by checking the deflection of the chain with the front of the engine off (if you open up the engine, it isn't that much more expensive to replace the part [about $40 for the two gears and the chain] - labor is the driver). There will always be a little slack due to gear play, etc.
Timing belt shouldn't cause changes in performance after the engine warms up either, that should lead to erratic performance such as backfiring during changes of stress on the chain (would lead to out-of-tolerance positioning of the camshaft and the distributor).
Yes, you could have a worn exhaust camlobe but that shouldn't cause back firing through the throttle body.  Cam health is best checked by removal and having it miked. Second best is to pop the valve covers and having the deflection of the rocker arms checked. At same time, inspection of valve spring tension can be accomplished. A worn camshaft can cause loss of performance, but it should be consistent.
Good luck
Bob

 

[PJ Stough]

Before I went to the trouble of removing all the stuff necessary to get to the timing chain, I would pull the distributor, and check the gear on it that engages the cam.  Back in the day those gears were known to wear, and I believe it would present the same symptom as a worn timing chain and gears.

I didnt see where you checked the plug wires.  Bad plug wires can cause cross firing, which would show up under power, such as climbing hills.

Just my thoughts.

Paul

 

[34footer]

Ok guys. I took it in for service. Got a call and they said i need a new timing chain for sure, and that they could move the harmonic balancer 1 inch before the rotor would move. I am going to go down there and have them show this to me. BUT, i also had a friend who is very knowledgeable about the 454 and said i could have a worn exhaust cam lobe.
What are your thoughts??
Ill give feed back from the service shop when i get back.
Things i have done so far: New fuel filter, new fuel filler cap, new rotor, coil, and distributer cap, and i have timed it.

One inch is not good, but not bad enough to cause your problem. Timing chains that are very much out of spec causes very hard starting. How many miles on your engine?
J

 

[gwcowgill]

Yes, you could have a worn exhaust camlobe but that shouldn't cause back firing through the throttle body.

An exhaust cam lobe worn could cause backfiring in the throttle body. As the cam lobe wears, it doe not release all the exhaust gasses into the exhaust manifold....when it becomes worn enough enough pressure is present (hot gasses) to ignite the incoming fuel when the intake valve opens. If you don't believe me try loosening up one of your exhaust valve adjusters!

 

[BobNSam]

An exhaust cam lobe worn could cause backfiring in the throttle body. As the cam lobe wears, it doe not release all the exhaust gasses into the exhaust manifold....when it becomes worn enough enough pressure is present (hot gasses) to ignite the incoming fuel when the intake valve opens. If you don't believe me try loosening up one of your exhaust valve adjusters!

I agree that it could if the camshaft were worn very badly. But if it was that bad, it should cause some other issues including missing, rough running, etc.
Anyhow, without further in-depth looking, $$$ , it is impossible to know for sure.

 

[phish555]

Bob, GW, Paul, and 34'r, thanks for all your imput. Let me give you the latest.
#3 cylinder is dead. Wont hold compression. THe shop is going to check the cam this afternoon. At this point we are looking at rebuilding BOTH heads, and of course the timing chain and gears. I think even if the cam isnt worn, i will have him replace it too while he is already there. He said the injectors were good. Right now i am sitting at 2675.00 and he said that is most likely on the high side.

 

[gwcowgill]

No matter what, it has to come apart. Your mechanic can check the cam l;obes by checking lift with the valve covers off. You never said how many miles are on your coach that I remember but I would be curious to know what is found and the mileage. The 454 has benn a pretty reliable engine over the years but they definitely do not like a lot of RPM. Hope it all works out good for you.

 

[phish555]

THanks GW,,,,59K

 

[gwcowgill]

59K is awful low mileage for internal engine problems on the 454. I have seen some though that had some lack of lube symptoms when they set for long periods of time. Definitely low mileage for timing chain problems.

 

[BobNSam]

59K is awful low mileage for internal engine problems on the 454. I have seen some though that had some lack of lube symptoms when they set for long periods of time. Definitely low mileage for timing chain problems.

I agree that this is low milage for a wear problem. Sounds more like a "catastrophic failure", like broken valve spring or badly burned valve.
Glad to here hear you have it all chased down and a path to fixin' it. I could wish it wasn't so pricey. Election to replace chain, camshaft, etc., while maybe not necessary could be a wise investment especially if you are planning to keep the beasty for a while.
Good luck and good travels.
Bob

 

[34footer]

Bob, GW, Paul, and 34'r, thanks for all your imput. Let me give you the latest.
#3 cylinder is dead. Wont hold compression. THe shop is going to check the cam this afternoon. At this point we are looking at rebuilding BOTH heads, and of course the timing chain and gears. I think even if the cam isnt worn, i will have him replace it too while he is already there. He said the injectors were good. Right now i am sitting at 2675.00 and he said that is most likely on the high side.

Now that you have found the bad cylinder it's time for a leakdown test to pinpoint the problem. The cam, timing chain, etc could all still be ok, may only be a burnt valve. what was the compression reading for cylinder #3? What about the others?
J