Air Bypass Valve - Exhaust Popping (1 Viewer)

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PabloCruise

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All,

I am having a popping in my exhaust upon deceleration, or btwn gears.

Vehicle in question is a '78 40 w/ stock 2F.

I believe my valve lash to be correct, recently set.

Pulling around 17" vacuum @ idle.

I am fully smogged, w/ air pump. I had heard that my problem may be my air
bypass valve going south. Mine looks original.

I had also heard that Ford had a similar part for cheap. I haven't checked w/
dealer, but SOR is asking $75 for a used ABV...

Anyone know if this diagnosis is correct?

Anyone know the Ford app that will donate the correct ABV?

TIA,
 
is that the thing that hooks up to the air injection manifold? if so they are like $20 from the dealer, i just got one for my 82 fj40.
 
oh it might be #5, I was talking about #25.

why did I get one? I have everything torn off the motor and for $20 I figured I stick a new one on when I had the chance. I thought I had a nice used air manifold but it turns out it was missing the heat sheild so I didn't use it and did feel like paying $200 for a new one.

John

PabloCruise said:
Well, I thought it was # 5 on this page...

http://www.sor.com/sor/cat048b.tam?xax=17917

But you are saying maybe it is #25?

Which one would cause the backfiring/light popping?

I am guessing there is a test for each.

Why did you get one Pimp, what symptoms?
 
John - I think you may be referring to the check valve that threads onto the air rail end. The ABV is a much more complicated component with numerous connections.

Above post sounds like my 02/80 FJ40 which had similar symptoms. In my case, it was a leaking connection between the EGR pipe and the exhaust manifold (there is a gasket there which had basically disintigrated and the pipe was rattling loose where it connects to the underside of the exhaust manifold on the DS near the firewall). You may want to check this out - you can see this and acess it by crawling under and looking up to the pipe/manifold connection - no need to take all the stuff on top off which is a big PITA. The other end of the pipe I am referencing connects to the EGR cooler on the bottom of the block near the front (radiator).

AndrewT
 
Like I said - I had the same noise on my 02/80 FJ40 which was an exhaust leak. It only happened when I reved the engine and then I would hear the popping sound when I let up on the gas and the engine decelerated. It also happened in between gear shifts as this is also when you let up on the engine rev.

It could be an exhaust leak anywhere. At the intake/exhaust manifold, at the heat-riser, where the exhaust flange mounts to the manifold, or where the EGR pipe mounts to the exhaust manifold (as was the case in my truck).

The pipe I am referring to is in the EGR section at SOR and you can see here - http://www.sor.com/sor/cat047.tam?xax=1047&page.ctx=cat047.tam. The pipe is item 2 and the gasket which can go bad is item 1.

Andy
 
Also - let us know if you have headers or not.

I replaced my EGR pipe with the stainless pipe that SOR sells and which is very good. It fixed my problem and no more leak though I rebuilt the whole exhaust and emissions system also in the process.

I found the EGR pipe gasket was basically gone and the pipe was only loosely connected to the underside of the exhaust manifold. You can check this easily by crawling under there and see if you can move the pipe where it connects to the exhaust manifold. I wouldn't tweak it too hard (compression fit on the other end of the pipe where it connects to the EGR cooler is a weak link and a PITA to replace if you bugger it) but just check. Also note that this is only a macro check. It could appear OK but only leak when under load - so see if you can listen down there when someone else is reving the truck for you.

Remeber - those pipes get hot so be careful what you touch down there if your truck is warm or running. Don't ask me how I know :rolleyes:

AndrewT
 
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Sounded like from the tailpipe as your exhaust leak disrupts the normal flow of exhaust thru the pipe.

AndrewT
 
I should also note I had a bad connection where the exhaust flange mates to the underside of the intake/exhaust manifold just below the heat riser. One of the studs here was gone and one other was stripped so only one stud there was holding it on and the exhaust flange donut gasket and the exhaust flange gasket were both essentially gone.

So, that could have also been the cause of my leak and popping sound. I never tried to narrow it down once I saw how both of these connections had deteriorated.

AndrewT
 
My rebuild did not include replacing the ABV, EGR valve, Smog Pump, or other major components as they were fine but not sure exactly what issue you have yet.

Just FYI. I would not guess that your ABV is bad at this point until you confirm whether or not you have exhaust leaks anywhere.

AndrewT
 
RustyNailJustin had this problem a few months ago [popping on decel]. I diagnosed a non-operational ABV, and he tested his. Sure enough, that was the problem. Sounds like yours too.
 
Justin posted a wanted ad for a used one! I check for him, and apparantly the new ones have gone obsolete. :mad:
 
Just curious - did you check for exhaust leaks? I know the easy thing to do sometimes is to just buy something new but you could be wasting money if the ABV is not the cause of your problem.

In my case, you can often have multiple problems so it takes a slow and methodical approach to start narrowing it down.

When I researched, I recall reading that ABVs typically go bad when the check valve off the air rail (that one between the air rail and the ABV) goes bad first. This is a simple test as the check valve should only allow air to flow one way so just disconnect and test. If check valve is good - this doesn't mean your ABV is also good. But if check valve is bad - this means your ABV is most likely shot also.

I sold my FSM Emissions manual with my 80 FJ40 so I don't have it here. The emissions manual tells you how to test the ABV if you want to check with others who may have it.

AndrewT
 
Thanks Andrew - I have not crawled under my 40 yet since this post went up...

Stuck in front of a 'puter it is easier to do some virtual wrenching, then when the weekend hits I'm all prepped for the real wrenching...

I have my emissions manual for my '78. 'Cept it is at home, I'll dig through it for all the troubleshoot.

This must be why so many people de-smog...
 
Check this out - some good info.

On 1975-81 engines, a belt-drive air pump supplies air to an injection manifold which has nozzles in each exhaust port. Injection of air at this point causes combustion of unburned hydrocarbons in the exhaust manifold rather than allowing them to escape into the atmosphere. An anti-backfire valve controls the flow of air from the pump to prevent backfiring which results from an overly rich mixture under closed throttle conditions. There are two types of anti-backfire valve used: 1971 models use "gulp" valves; 1972-84 models "air by-pass" valves.

A check valve prevents hot exhaust gas backflow into the pump and hoses, in case of a pump failure, or when the anti-backfire valve is not working.

In addition all 1975-82 engines have an air switching valve (ASV). On engines without catalytic converters, the ASV is used to stop air injection under a constant heavy engine load condition.

On 1975-81 engines with catalytic converters, the ASV is also used to protect the catalyst from overheating, by blocking the injected air necessary for the operation of the converter.

On all 1975-82 engines, the pump relief valve is build into the ASV.

On 1982 and later carbureted engines, the air injection system incorporates a "feedback" loop. An oxygen sensor threaded into the exhaust manifold monitors the oxygen concentration in the exhaust gas, and indirectly signals the air pump to divert compressed air to either the exhaust ports or the air cleaner, depending on the oxygen levels in the exhaust. On 1983 and later engines, an EACV (electronic air control valve) serves as gate keeper for the air flow according to engine temperature. The EACV takes the place of the ASV on earlier engines. Thus, the feedback system constantly readjusts itself in order to reduce hydrocarbon, carbon monoxide and nitrogen oxides in the exhaust. If the catalytic converter overheats during operation, a converter-mounted sensor will temporarily shut the air injection system off.
 

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