Engine intake backfire

[cmiulius]

Hello everybody,

I have a big problem which means backfire in the intake. I had it before i replace the timing chain, i have it after. New air mass meter, new distributor, new injectors, spark plugs etc. Starts and idle OK. The backfires are not always there. so what else can be? Any advice is more than appreciated. Thank you very much.

[cozye]

Hello everybody,

I have a big problem which means backfire in the intake. I had it before i replace the timing chain, i have it after. New air mass meter, new distributor, new injectors, spark plugs etc. Starts and idle OK. The backfires are not always there. so what else can be? Any advice is more than appreciated. Thank you very much.

What year/model car.

What is your fuel pressure ?

What vacuum are you pulling ?

What temp is the motor running?

Why did you replace your timing chain?

What timing are you running?

Have all vacuum lines and intake leaks been inspected and ruled out?

[cmiulius]

I have a 76 280z automatic for now. Fuel pressure is 3.5, I don't know about vacuum, engine temp looks normal. Actually I did not replace the chain, just the rails and the tensioner. I install the timing by the book, on number 2 hole on the cam sprocket and the crank on TDC. Also i aligned both with the chain marks. Mystery so far.

[cozye]

Fuel pressure should be about 30psi at idle with the correct vacuum. Not sure what scale you are measuring 3.5 on?

You will need a vacuum gauge and a good vacuum measure. You need to rule out any intake and vacuum leaks as well as verify correct operation of fuel supply/pressure before moving on.

[FastWoman]

Maybe you're running carbs on the car? I wouldn't think an EFI car would even run on 3.5 psi. If you've got EFI, then either you've got a bad fuel pump, bad fuel pressure regulator, or clogged up fuel system. That would make it run very lean and would cause your intake backfires.

[cmiulius]

sorry guys my mistake. that dot doesn't belong bet. 3 and 5 so is about 35.

[cozye]

well get a vacuum reading at idle.

And what is your fuel pressure when you rev the motor up ?

[cmiulius]

As I sad before, I have fuel pressure I have vacuum, what I was thinking is that may be a possibility of having a lazy valve that has a delay on closing? The car was sitting in a garage for 13 years. weeks ago when I rev the engine one of the tappets in the end of the valve stem jump from his place, which is not normal.What do u guys think about it?

[FastWoman]

Well, you might be running lean, or you might have some other problem, but if your valves are sticking, you'd better fix that first, before something awful happens. You can clean up the lower parts of your valves with SeaFoam treatment in the intake, and you can clean up the upper parts and internal engine parts with a Gunk flush immediately prior to changing the oil. A lot of people get very nervous about motor flushes, but I've always had very good luck with them, particularly when treating engines with sticky, goo'ed up valves. One precaution: I wouldn't use a motor flush if there's lots of crusty, polymerized, black sludge in your engine. A safer approach for a sludge-filled engine is to use high-detergent motor oil like Chevron Delo (formulated for diesel engines) or to add Rislone (a detergent additive) to your motor oil.

After you get your valves operating freely, THEN fix the other problems.

[cozye]

As I sad before, I have fuel pressure I have vacuum, what I was thinking is that may be a possibility of having a lazy valve that has a delay on closing? The car was sitting in a garage for 13 years. weeks ago when I rev the engine one of the tappets in the end of the valve stem jump from his place, which is not normal.What do u guys think about it?

I asked for specifics on the fuel pressure, because the fuel pressure is a variable. Just because you have 35 doesn't mean its good. You could have a faulty fuel pressure regulator. The stock EFI is very sensitive to fuel pressure. Car not running, but fuel pump on should be around 36-38. Fuel pressure at idle should be 28-30. Fuel pressure at low vacuum like when you first stomp on the gas should be 36. If you are getting 35 PSI fuel pressure at idle with 18hg of vacuum, you will be running too rich and you might have a faulty FPR.

Alternatively, fuel pressure at idle could be good, but when you idle up if fuel pressure drops that would indicate a partially blocked fuel line or pickup, etc.. that would starve the system of fuel under load.

That being said, your new information regarding valve train needs to be addressed. Do what Sarah said, also after you get done with your top end cleaning, do a valve adjust with motor hot.