can a worn cam lobe do this?

[Oldskool153]

ok so most of u guys know ive been complaining about the straight pipes on my car popping under heavy acceleration. well my corvette mechanic (a 20 year gm tech that now works for a place called contes corvettes near my town, hes the best guy i know with these cars) is gonna put in on a scanner hopefully this weekend, but he said something to me yesterday that has me worried> i told him that ever since i put straight pipes onit it will now sometimes pop under hard acceleration on the 1-2 upshift (cars an auto) anyway he told me not to worry and that this probably isnt the case but that he has seen cars with a worn cam lobe backfire threw the intake under heavy load but run fine during normal load. now im almost 100% sure mine was doing it threw the straightpipes (again never had the prob till i put them on) but my minds still wondering "what if it is my cam?" how hard is it to change my cam? or am i worrying too much?

 

[vetteoz]

he has seen cars with a worn cam lobe backfire threw the intake under heavy load but run fine during normal load.

Very feasible but roller cams seldom drop cam lobes
Have pulled very high mileage cams out and there has been little if any wear
Tune ( going lean ) still looks to be most probable cause of problem

 

[vetteboy86]

What is the rest of your exhaust setup? Any cats? My guess is that the car is running rich due to the straight pipes. Are you running stock O2 sensors?

 

[Schrade]

Can you measure lift with a mic, and the V/C pulled off, spin the block by hand?


edit:

 

[Oldskool153]

car is a 92 auto with 85k she had 82k when i bought her in sept, complety stock except for straight pipes, hollowed kats. never had a backfire issue until the straight pipes were put on her (still only backfires on the 1-2 upshift while floored no other gear change) today i romped on her alil and the sound is deff comming threw the pipes because i can feel the pipes vibrate alil as it back fires and i can hear it comming threw the pipes, which makes me feel alittle better b/c he told me if it was the worn cam you would hear it threw the intake not the pipes?

 

[Oldskool153]

also yes the stock 02 sensors are still in there

 

[John Robinson]

Noise

On my 93 auto I removed the resonator and I get popping (backfire)when I engine break or just rev it up in neutral. I suspect that if I did not have any mufflers then it would be louder.

I would not worry about it if the car is running good. After all the purpose of running a straight exhaust is to have noise. Remember you have reduced the back pressure and The ecm has lost some of it's program parameters.

Check out this service bulletin
http://www.corvetteactioncenter.com/kb/questions/263/1992%3A+Service+Bulletin%3A+Engine+Backfire+During+W.O.T.+1-2+Upshift

Read this it is specific to 92 auto vette and it covers the issue of backfiring on wot. It list several different sensors to be used to eliminate the backfire.

 

[89blackvette]

You say it is popping through the pipes and it did not pop before you put the staight pipes on it. By process of elimination it is the pipes!! Put a cross over pipe up close to the engine, or if headers close to the headers and it may solve your problem. It is probably gas accumilating in eexhaust during shift that is then detonating causing a pop. Bad cam would have made some difference in runnung before you put the pipes on it. Go to your muffler shop first. Some of these highly trained mechanics over anaylize situations and try to find some strange problem instead of just listening to the customer. GOOD LUCK, LET US KNOW THE OUTCOME.
Mike

 

[Hib Halverson]

Provided this problem only occurs at the 1-2 shift, I'm going to suggest that the engine has been doing that for a long time. You just never noticed it until you added straight pipes which not only makes it much louder but may tend to increase the popping.

I think the first thing you need to do is reference that TSB listed earlier. Problem is, getting that replacement cal is going to be tough. I'll guess that that PN has been discontinued, but check with a dealer anyway.

As for your tech suggesting a cam lobe going flat...that's possible but unlikely on a roller cam motor that old and having stock valve spring pressures. Nevertheless, you can easily check valve lift with a dial indicator and rolling the motor over by hand.

 

[G Winter]

On my 90 I noticed a popping in the exhaust when decelerating and shifting after I took the stock mufflers off. I think this was due to poor fuel mixture,, to rich maybe to lean.
After I replaced my injectors with the BoschIII from FIC that went away . Now has a smoothe exhaust note. I believe the old OEM injectors were what caused the popping .

Glenn

 

[Oldskool153]

update! bigger fish to fry!

yea thanks for all the help im sure the sound is threw my pipes not my intake, my mechanic had told me beofre that he highly doubted it was the cam and that if it was i would hear it threw my intake not tailpipes. But once he said that my mind automatically started worrying. Anyway now i have bigger fish to fry i had the car out all day yesterday and it ran fine...i looked down and noticed my car was almost out of gas (to the point of almost running out, to the point of sputtering) so i stopped and put $30 of sunoco ultra 93 octane..with in 5 mintues my car started runing very poorlym stalled out 3 times with in a half hour. would idle fine but wuld miss and run poorly under acceleration..i let it sit overnight and presto this morning shes fine. im thinking maybe i got bad gas so i put dry gas init and today i put over 100 miles on her and shes been fine. possibly bad gas? she never once threw on a checkengine light. if not bad gas what are the odds that running it so low on gas could have sucked up some dirt but then sucked it threw and is now fine?

 

[John Robinson]

Latest problem

yea thanks for all the help im sure the sound is threw my pipes not my intake, my mechanic had told me beofre that he highly doubted it was the cam and that if it was i would hear it threw my intake not tailpipes. But once he said that my mind automatically started worrying. Anyway now i have bigger fish to fry i had the car out all day yesterday and it ran fine...i looked down and noticed my car was almost out of gas (to the point of almost running out, to the point of sputtering) so i stopped and put $30 of sunoco ultra 93 octane..with in 5 mintues my car started runing very poorlym stalled out 3 times with in a half hour. would idle fine but wuld miss and run poorly under acceleration..i let it sit overnight and presto this morning shes fine. im thinking maybe i got bad gas so i put dry gas init and today i put over 100 miles on her and shes been fine. possibly bad gas? she never once threw on a checkengine light. if not bad gas what are the odds that running it so low on gas could have sucked up some dirt but then sucked it threw and is now fine?

I don't think you pulled up any dirt but I do think you probably found the water that forms in the tank when it sits with a low tank in humid weather. The tank is a micro environment and water will condensate in the tank. You have your own answer in the fact that you used a water dissipater and it runs fine now. Be aware that if the gas you buy has water in it from there tank and it is of sufficient volume it can toast an engine in a few blocks. Don't laugh I have seen this happen. One case I remember is a police car that filled up his tank and drove less than three blocks and the engine quit. and while he was waiting on a tow 3 more cars stopped near him after filling up at the same station. When the tank was pulled and drained it had almost a full gallon of water in it.

 

[Oldskool153]

yea the only thing that doesnt add up is, i drove to my local corvette shop (about 30 miles) to buy the dry gas. it ran fine all the way there but was running poorly yesterday after filling up. i let it sit overnight and shes been fine all day. i still added the dry gas just to be safe. any ideas as to why it would run bad lastnight, sit over night and run fine today? thats why i had assumed it was dirt in the fuel filter or something more then a water issue. eitherway i should keep a better eye on my gas gauge

 

[Oldskool153]

and i do beleve it was related to the gas in someway because with in 5 minutes of filling up from the empty tank she wasnt running right

 

[89blackvette]

These darn cars are like a woman. fine one minute and a mess the next. Thay require some petting and attention, a new little piece of shinny stuff helps too. I hate mine once in a while but she seems to see my displeasure and straightens out on her own. Damdest cars I have ever owned. I think they have a bottle of CORVETTE PMS in Bowling Green that they put in them.
Mike

 

[G Winter]

You stirred up the water and it had settled down to the bottom again. Without the dryer you would have the same problem the next time you fill .

Glenn

 

[Oldskool153]

ahh that deff amkes sense now. i picked up a new fuel filter and a new air filter from pep boys tonight, will prolly put them on tomorrow after work depending on how tired i am (im a 23 year old building inspector and have to be onsite by 630 on a sunday morning. i went out tonight and am going to stay up all night and go straight to work) anyway if im not too tired after work tomorrow im going to put them on< she has 86k so i figure it cant hurt to change them eitherway. amd yes these cars are DEFF like women without a doubt lol

 

[Oldskool153]

and also the stiring up also makes sense because i normally only put 10 at a time in, yesterday i put in 30 at one time then she started running poorly, so im sure it deff stirred something up