The exhaust manifolds each have a metal tube that run to a part that's on the drivers side of the intake. I think it's the air bypass valve.

The van drives fine up to about 25 mph, above that speed, it acts like it's starved of gas, and there is a sound like a small exhaust brake coming from the air bypass valve. It sounds like there's a blown diaghram in there, and I can feel air pressure blowing out of a hole in the thing at the same rythm as the exhaust brake sound. The sound and the loss of power always happen at the same time.

Is the valve likely to be the problem? Or is the valve doing it's job correctly, and reacting to a stuck egr valve or something?

The problem may have started during a time of overheating, due to a leaking radiator, while be driven by a band of 20 year olds who thought that since the coolant overflow tank was full, that all was fine.

Any tips or advice will be greatly appreciated, as I will be travelling to work on the van in a remote location, and diagnosing this problem is beyond my ability.

Thanks.

 

A little more info...

The exhaust brake sound started, quietly at first, a couple of days before the loss of power symptom. If there is a diaghram on the bypass valve, maybe it had a small tear before blowing open enough to cause major trouble?

Also, the radiator was repaired and and eng temp was good for about 200 miles before the loss of power happened, so the overheating may not be related.

 

There isn't supposed to be any exhaust escaping from that valve or any other part of the Thermactor(air injection system)so that is definitely a problem.. there cannot be any exhaust leaks upstream of the O2 sensor or the engie will run excessively lean. If you don't have any smog tests where you are I suggest stripping the whole works off the motor, plug the holes in the manifolds and cap the vacuum lines attached to the diverter valve/s. This system does not play any part in the EFI control system and can be deleted without affecting it. If you don't want to do that then I suggest you pull the doghouse off and find out exactly what part is broken and replace it.

 

I have pulled the dog house off, and I haven't found anything that is broken. I do know that the ABV is making the sound at the same time the power cuts out.

I'll look into emissions testing here, and maybe pull the whole thing.

 

Turns out the state of WA wants this van to run clean. Any ideas for testing or diagnosing? Will the codes tell me anything about parts related to this issue?

 

It never hurts to pull the codes to see if anything shows up.