1998 Mazda B2500 Vacuum Leak

bmassa · May 12, 2017

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 · May 12, 2017

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

bmassa · May 12, 2017

Thank you RonD.

I will give that idea a shot.

bmassa · May 13, 2017

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 · May 14, 2017

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 · May 14, 2017

Got it. Again, thank you.

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

bmassa · May 23, 2017

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 · May 23, 2017

Pull PCV hose and plug that intake port with your finger

Tenper616 · Jan 5, 2024

RonD said:
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

I'm having the same issue, assuming I pull the iac connection and it drops the idle down, you said no leak, but what can cause a vacuum leak with out a vacuum leak? I've checked every line I can think of, I unplugged the iac and it dropped about 200 rpm, and I'm completely stumped

RonD · Jan 5, 2024

You can only do that test after engine is warmed up and idling
1998 2.5l Manual trans should have warm idle of 700-750rpms
If you unplug the IAC Valve it should drop down to 500rpm(barely running) or stall either means no vacuum leak

Tenper616 · Jan 5, 2024

RonD said:
You can only do that test after engine is warmed up and idling
1998 2.5l Manual trans should have warm idle of 700-750rpms
If you unplug the IAC Valve it should drop down to 500rpm(barely running) or stall either means no vacuum leak

Okay, so my b2500 2.5l manual trans idles at around 950-1000 and after driving home from work, about 30 minutes, I pulled the iac and it dropped to around 700ish

RonD · Jan 5, 2024

Then the computer is setting the high idle
Not sure why

The CEL(check engine light) would be on if its was the ECT sensor, and MPG would be way way down
P0116 Engine Coolant Temperature Circuit Range/Performance Problem
P0118 Engine Coolant Temperature Circuit High Input

CEL would also come on if computer couldn't lower the idle more
P0507 Idle Control System RPM Higher Than Expected
So computer wants it that high

Could be the Computer "thinks" your foot is on the gas pedal all the time
That's the TPS(throttle position sensor)
Its a 3 wire sensor on the throttle body, its a 5volt sensor that tells the computer the position of the "gas pedal", the throttle plate
It sends 0.69 to 0.99volt to computer when "foot is off the gas pedal", idle
And 4.5volts when "gas pedal is to the floor"

After engine is warmed up shut if off
Unplug the TPS 3 wire connector and start the engine
CEL should come on pretty fast, sanity test
See if idle is lower

Regardless, check the inside of connector and TPS for corrosion, also tug on each wire to make sure it's being held tight in connector

Shut off engine, plug connector back in
Start engine
CEL should go off or not come on again

Tenper616 · Jan 15, 2024

Other issues I have is when I change the air vents it takes forever to actually move them like a vacuum leak, I've replaced/checked the TPS already with a new one and checked the wiring for it as well, when I let the idle get to about 2700rpm it acts like it gets either way too much fuel or not even almost enough, depending on the temp

1998 Mazda B2500 Vacuum Leak

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