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1998 Mazda B2500 Vacuum Leak

bmassa

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I have 1998 Mazda B2500 2.4L 4 cylinder manual transmission pickup that has a vacuum leak that I cannot find. The engine has roughly 255,000 miles on it.

Symptoms:
Once the truck start and is warming up, if I rev the engine above 4000 rpm's, after I release the throttle the engine dies. I then start the truck again and it idles very rough. If I let the engine completely warm up, then I won't have this issue. Also, I disconnected all the vacuum hoses from the engine, minus the intake from the air filter, and hooked up a vacuum tester directly to the engine. The needle was vibrating 1-2Hg around the 19Hg mark.

It is not throwing any codes and it is passing SMOG though it is consuming more gas than before.

I have tested all the vacuum hoses coming from the engine and they have suction.

Any ideas?
 


RonD

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Welcome to TRS :)


After engine is fully warmed up and idling, unplug the two wire connector on the IAC(idle air control) valve, it will close
RPMs should drop to 500 or so, or engine may even stall
If it does then no vacuum leak

If idle stays high then you do have a leak, recheck hoses with IAC valve unplugged, RPM drop would indicated a problem with that hose or device at the end of that hose
 
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bmassa

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Thank you RonD.

I will give that idea a shot.
 

bmassa

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RonD

After letting the pickup warm up their was no change in rpm once I disconnected the IAC valve two wire connector. So I am back to trying to find the vacuum leak.
 

RonD

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Leave IAC Valve unplugged from power and pull one vacuum hose at a time off of the intake manifold, and plug the port in manifold with your finger, if RPMs drop then that hose has the leak.

Start with Power Brake booster hose and then PCV Valve hose, they are the largest vacuum hoses
 

bmassa

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Got it. Again, thank you.

Is the IAC Valve going bad a common problem for these types of pickups?
 

bmassa

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So I disconnected the IAC and began disconnecting vacuum hoses. Every large hose I disconnected caused the engine to bog down and want to die, except the one connected to the PCV valve. I pulled the PCV and the truck remanded idling as if nothing had changed. Recently, I changed my PCV valve and the hose visually looked fine. I am stumped again.
 

RonD

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Pull PCV hose and plug that intake port with your finger
 

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