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Vacuum Leak


Well-Known Member
January 17, 2008
Reaction score
City, State
Long Island, NY
Year, Model & Trim Level
1994 GMC Yukon 2 Door
Hi everyone sorry I've been asking so many questions lately. Well anyway my 92 X has always ran great but I have always had a problem with the dreaded pinging. Right now I have the octane shorting block out which helped the pinging but didn't eliminate it. I have always suspected that the problem was a vacuum leak but thought that it was probably the intake gasket. Today I found what is probably the cause of all my troubles, a vacuum hose that broke clean off! It is the hose on the vacuum tree that is attached at a right angle and is on the front part of the vacuum tree. What does this go to? I thought I remembered hearing that it goes to the FPR but then wouldn't the truck run bad? I can see the rest of the hose which is bundled with the other hose that enters at a right angle but then it goes off towards the passenger side and I can't see it any more. Since the truck runs good can I just plug this hose? If not how hard will it be to replace this hose? Is there some way I can splice the ends together? Right now I have the hose plugged until I figure out what to do. Any help with this matter would be appreciated. Thanks a lot, James.

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Thanks for the suggestion devilboy. That suggestion is exactly the kind of repair I was looking for. By some stroke of luck I had some hose that fit perfectly laying around. So now both ends are back together and I don't think they are leaking. I still have to take it on the highway to see if this stopped the pinging though. Now does anyone happen to know what this hose is connected to? I am just curious to know what hasn't been connected for so long.

It was the side farthest from the firewall.

Hmmm....I don't have a hose there. There's a spot for one, but it's not hooked up. Must be one of the small differences between older and newer first gens. If it goes to the passenger side, it could hook up to the FPR, the A/C, or the sensor on the air box. Even if it's the FPR, it's possible to not notice a difference if the Fuel Filter is plugged up enough to drop the pressure. My FPR didn't used to do anything, but it was because I had 0 psi of fuel pressure at the rail. Thing is it still ran fairly well.

if its like mine it goes to the egr valve diaphragm

No, it's not the egr because since it is a 92 there is no egr. Oh well it doesn't really matter what it goes to. Now that it's connected though the truck runs exactly the same so I think it goes to the connection on the air box which I have plugged anyway because I have a KKM intake. However I took the car on the highway last night and it still pinged even though it was reduced. This is really bugging me, what could be causing this pinging? I've done a tune up with plugs, wires, filters and I have cleaned the MAF a few months ago. Also I just ran seafoam through the brake booster. This is with 89 octane and the octane shorting block out. Next on my list to do is torque the intake plenum and replace the O2 sensor. Any ideas on what else I should try? I think it may be the lower intake gasket, however my dads friend who is a mechanic recommended not to change this gasket because it is a high mileage engine. I am not sure I agree with this, what do you guys think? Also it's not the head gasket because there's no bubbles in the radiator or foam on the oil cap. The car runs great except for this but I would still like to fix this problem because the gas mileage with the octane shorting block out is killing me. Thanks for everyone's help, James.

there should be a diragram on you rad support to tell you where all the hoses go to.
i do recall reading some where before that a higher octane gas can cause pinging as the 4.0 only calls for 87 octane gas.
did you replace the plugs after the sea foam ? if you did it before you could have carboned the plugs. as for any gasket if it needs to be replaced cause of leekage or age ,change it.
also check your timming even tho its not adjustable it could be out.the spout could have came lose, iv seen that before.

Thanks, how would I check the timing, I don't think there's a timing gauge? Also is the spout plug the same as the timing pill? That's the gray plug near the passenger headlight right? I'll check that but I was bored a few weeks ago and did a KOEO and KEOR test and the only code that came up was the octane shorting block circuit open.

you can check it with a timing light just like you would any other motor. remove the gray plug an look for marking on the front cover an lower pully. whte out works good for high lighting them. start the motor an see if it shows 10* btc if it does its right.

What does the sensor/vac hose on the airbox do?
My air box has a crack in it and my CEL comes on a few minutes after I start the X.