camaro_junkie

This question goes out to those with a 5 speed. I have this backfiring problem with my car. Every time I rev the engine up to 3000 or so rpm, and then shift, it backfires. Not sure what it is. I'm pretty sure it's not the MAF sensor or the intake from the filters to the throttle body. It could be the EGR, or maybe crossfire on the spark leads, or the timing chain, or the deceleration valve. Anyways, I was testing the deceleration valve, and according to the camaro shop manual, if you take the vacuum signal off and then put it back on, the rpms should drop and the valve should open for at least one second. One second is pretty hard to measure, but my guess, it was only 1/2 second that it stayed open. My question is, is there a definitive test to see if the deceleration valve is bad?

Doward

Deceleration valve?

Wtf?

Please describe...

camaro_junkie

If you have a 2.8L with a 5 speed, you probably have a deceleration valve. It's there for the same reason as the A.I.R. pump (for down shifting). It's that small black cylinder with two medium size hoses and a small hose coming out of it. It's mounted on a bracket coming off the alternator

Nixon1

Actually the A.I.R. pump is for emissions control..... (Also referred to as the smog pump)

Are you referring to the check valve in the smog pump? It's a trick one way valve..if the valve is blown, it will allow air to move past it in the direction it's not designed to, and it will damage/destroy your smog (A.I.R.) pump... A physical way to test the valve...but it's kinda gross, and it has to be removed in order to do this...is to blow into it as hard as you can. You should only be able to blow through it one way.... (Obviously)

camaro_junkie

Yes, the A.I.R. pump is for emissions, but the reason it's there is because of the down shifting that is done with the manual transmission.

But, no, I'm not talking about the check valve. I already had a good time with that part a couple of years ago. It was staying open and it burned the rubber hose that connects the check valve to the pipe from the pump. When I tried to take it off, the threads were screwed, so I had to pull out that whole three-pipe assembly.

Anyways, if you have a 5 speed (as I assume you do, since you replied to my post) you will see this valve mounted on a bracket coming off the alternator. It allows intake air to bypass the throttle valve when the throttle closes and creates a high vacuum. It's a small black cylinder about 1" in diameter and 2" long, you can't miss it. But since no one seems to know what I'm talking about, it doesn't look like anyone will be able to help me.

Nixon1

I'd look, but....I HAD an automatic Camaro, and I don't have my Camaro anymore.... Here's the sig for ya.

drdave88

iTrader: (3)

i have the exact same problem. if you find a fix for it, let me know cause im sick of puttng $$$ into it.

Doward

Ok, we're talking about this, right?

Doward

Pic:

Nixon1

You know it took me almost a full minute to spot why your engine bay looks so.....easy to access. No EGR! And that valve thing was definitely not on my car....and I dont recall seeing it on auto 2.8's or 3.1's, so it looks like it is exclusive to 5 speeds...

Doward

#1) PCV Actuation Valve (Don't know the exact name )
#2) T Fitting for vacuum source
#3) IAC connection
#4) IAC

In a nutshell - you do know the pcv system, right?

Positive Crankcase Ventilation. if your engine were totally sealed, it would actually pop due to built up pressure from various things - air heating up inside the engine, oil vapors from heat, all sorts of things. Basically, prevents your engine from getting bloated

Ever seen a PCV valve? It's a one way check-ball valve. When pressure builds up enough, the ball comes up, and the pressure is released.

The vacuum valve helps that, since the RPMs stay up in a stick shift when you shift (air pressure in the plenum has to be pulled through still) and pressure spikes up. The vacuum valve senses the vacuum pull, and helps pull the crankcase vapors out.

It has nothing to do with the AIR system. the AIR system's ONLY purpose is to inject extra oxygen after the exhaust comes out of the head, to help light the cat quicker for emissions. That's why I removed my AIR system after I removed the cat.

TomP, you want to back me up on some of this?

Doward

Its not on automatics, because your auto tranny shifts under WOT. If everyone power shifted, we wouldn't need it either, lol!

Nixon1

Shows what I know about 5 speeds huh? NOTHING!!

Doward

Which is EXACTLY why you need a 5 speed in that 'stang!!!

Nixon1

5 speeds are slow. (Compared to race automatics, that is. Speaking of....check out www.lentechautomatics.com, look at the Street Terminator transmission. Ohhh yeah, give me like 3 or 4 months and that baby is MINE!)

BackInBlackGP

This is what you are referring to, right?

camaro_junkie

Doward circled it in the first picture he attached. That's the deceleration valve. And as I said before, it's on the 5 speed, because when you downshift the throttle valve is closed and the vacuum in the intake is too high, so the deceleration valve allows air to bypass the throttle valve to decrease the vacuum.

But whatever the function, the fact remains that it can cause backfire if it's not operating properly. My question was, what is a definitive test to see if it's operating properly? I'm going to replace the valve anyways, I can get it for $13CAD. I just want to know out of curiosity. BTW, for anyone looking for this valve, no one makes them anymore, but there is a warehouse in the US that has stock, so you can get it through Chev.

camaro_junkie

Look a little to the left BackInBlackGP.

Doward

Actually, no, it doesn't decrease the vacuum - it actually uses that vacuum spike to help excavate the crankcase of all pressure.

Sorry, I don't know who told you it can cause a backfire -- but they are wrong. All it does, is help the PCV system. Trust me - I took it off, and plugged the pcv system to find out what it did when I was removing the smog crap. I decided the PCV system and teh Evaporative Gas Canister were two items I'd keep, as the PCV system is important.

Check your wires, plugs, and FIRING ORDER. #1 cylinder is the front PASSENGER side (I set my plugs according to #1 being the drivers side - it had your problem, as well)

Check your timing. Check your cat. Those can cause backfiring.

Where exactly are you hearing the backfire?

camaro_junkie

Sorry, that was a misquote. I'm pretty sure it does decrease the vacuum, but what it's actually meant for is to lean the a/f mixture when the throttle closes and the manifold is at high vacuum.

This translates to YES it can cause backfire. Trust me. I have the definitive source, just short of the people who designed the car.

An as for where the backfire is coming from. Who knows! Too hard to tell.

Doward

Think about that for just a second...

Computer is feeding enough fuel for 3k rpms at full tps voltage (batch fire here) so a bit of the fuel is pooled behind the valve when you let off the throttle.

That gas gets sucked in, with a LOT less air - quick rich condition. The computer has compensated by now so all is well again.

If what you say were to be correct, then it would lean out the a/f ratio whenever you were cruising the highway (little throttle, high vacuum). Why doesn't it? The O2 sensor will tell teh computer to add fuel if more air is introduced (as you are saying with the p/t - hi vac scenario)

You don't have to believe me. Screw what the book says - follow the pipe yourself. Pull off teh pipe, plug it up (don't need a vacuum leak) and go for a spin - see if the backfire leaves.

I'm 99% sure your backfire is being caused by too much fuel not being burned in the cylinder - check your spark.

camaro_junkie

First off Doward, I respect your opinion. Don't think I'm getting pissed off at you or anything. This is just a friendly conversation, not an argument.

Secondly, WHATEVER! Who cares what that thing does. I just want to know how to check to see if it's operating properly no matter what the function of the thing is (besides replacing it).

Doward

lol, np np... we're all here to learn, right?

If when vacuum is applied, it opens, then it's working - you said it yourself. It's just a vacuum activated valve, to the best of my knowledge,

BackInBlackGP

I was referring to the check valve, not the deceleration valve...

camaro_junkie

Doward: I'm assuming it works on a spring, so even if it opens, it may not stay open long enough to do whatever it's supposed to do.

BackInBlackGP: I just meant that we're not talking about the smog pump check valve at all.

Doward

If the spring were weak/broken, it wouldn't close again against the vacuum (even idle vacuum which should be around 18-20in).

Since it does open, and it does close... it's working fine

camaro_junkie

The spring can get fatigued though. That's why you need a different test to see if it's working.

BackInBlackGP

Could you use a vacuum pump to actuate the diaphragm/valve and another air supply to see if the valve itself flows air when it's supposed to?

camaro_junkie

I don't follow...

BackInBlackGP

Suck on the actuator port , and see if air is allowed to flow through the valve passage.

BackInBlackGP

I know. I was simply clairifying the location of the smog pump check valve...

camaro_junkie

That test will determine if it is working at all, but not if it is working properly, for instance if the spring/diaphragm is weakened.

Doward

It's $13. If it's worrying you that much, just replace it.

camaro_junkie

We should just have a conference call.

Yeah, I will just replace it anyways, but what can I say... I just have to know.

malaka

iTrader: (1)

can any one post a link of where to buy this deceleration valve?

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p/n (at least for my 1988 v6) is 14089246, AC Delco 214-453