Weird Vacuum / idle issue
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4wd Low
Joined: Nov 2023
Posts: 13
Likes: 5
Weird Vacuum / idle issue
New to me 2002 V10 2wd Excursion that ran and drove great, but the HVAC was stuck on defrost when I purchased it. I followed the vacuum lines and found that where the line from the reservoir attaches to the intake behind the throttle body it was disconnected resulting in no vacuum to the hvac system. Now I would have assumed that this would have caused all sorts of running issues, but the engine ran fine with the line disconnected even though there was a huge vacuum leak.
Once I connected the line, the engine will not longer idle. If I hold the throttle open manually it will run, but it dies as soon as it's let go. I played around with the IAC valve figuring it was stuck closed and someone popped off the vacuum line to allow air to bypass the throttle plates, but the valve was clean and the plunger moved back and forth manually.
Here's where it gets weird. If I pop off the IAC line that goes to the air intake tube, it's sucking in air very strongly and the engine will run with that hose disconnected from the air intake, once connected the engine dies. The air filter is clean and there are no obstructions in the air intake tube. Basically, with the system connected as it should be, the engine isn't getting enough air past the iac to run the engine, but there is a ton of suction on the line so I know the IAC is opening. This makes no sense to me.
Could it be that even though I feel vacuum at the iac, it's not opening quite enough to run the engine?
Any suggestions? TYIA
also posted in the X forum.
Once I connected the line, the engine will not longer idle. If I hold the throttle open manually it will run, but it dies as soon as it's let go. I played around with the IAC valve figuring it was stuck closed and someone popped off the vacuum line to allow air to bypass the throttle plates, but the valve was clean and the plunger moved back and forth manually.
Here's where it gets weird. If I pop off the IAC line that goes to the air intake tube, it's sucking in air very strongly and the engine will run with that hose disconnected from the air intake, once connected the engine dies. The air filter is clean and there are no obstructions in the air intake tube. Basically, with the system connected as it should be, the engine isn't getting enough air past the iac to run the engine, but there is a ton of suction on the line so I know the IAC is opening. This makes no sense to me.
Could it be that even though I feel vacuum at the iac, it's not opening quite enough to run the engine?
Any suggestions? TYIA
also posted in the X forum.
Laughing Gas
Joined: Dec 2019
Posts: 893
Likes: 255
From: San Diego, CA
I'd probably start by disconnecting the battery for a bit and see if it resets itself. I'm assuming no check engine light, but it would be worth attaching a code reader to check if there are any stored codes. I'd be looking at MAF values and O2 sensor readings.
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4wd Low
Joined: Nov 2023
Posts: 13
Likes: 5
I'll try that. I've ordered a new IAC and PCV just for good measure. No CEL present at this time. I have a cheapo scanner I plugged in and nothing was stored, but I can try again after I reset the battery and attempt to drive it a little bit.
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4wd Low
Joined: Nov 2023
Posts: 13
Likes: 5
Before installing the new IAC, PCV, and MAF that I ordered I gave your suggestion a shot and after leaving the battery disconnected for an hour it fired right up and idled almost immediately. I did a short drive around the block and it seemed fine!
Thank you for that suggestion.
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