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Old 05-22-2021, 10:49 PM   #1
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Default Chasing vacuum leak still

Ok guys, I've looked high and low. Took the vehicle to a mechanic where they replaced my IAC valve, whether it was bad or not I do not know. But here's my issue:
Firstly, I have no engine codes except occasionally fuel trim lean but it rarely shows up. At idle, the truck will drop RPM from like 8-900 down to 5-700 for 5ish seconds and then recover. It only does it at idle, only after the truck is up to running temp and I can visibly see with a vacuum gauge that the vacuum drops from about 22-23 down to about 18-19. So there's a leak, but it seems to get bad when something opens, something closes, switches on or off.
I'm in your hands guys. 1997 hardbody 4cyl 5 speed manual. I've got a OBDII tool, fluke multimeter, vacuum tester, even made a smoke machine that led me nowhere. I'll run any tests you guys want.
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Old 05-22-2021, 11:05 PM   #2
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Get some starting fluid and spray the hoses. Also do the intake and t/body gaskets.

Why the engine revs, you found a leak, don't stop at the first sign either.
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Old 05-22-2021, 11:17 PM   #3
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I did that but I didn't find anything. The mechanic I took it to also replaced the throttle body gasket. But I did the starter fluid trick with no results. I'll repeat it once more tonight just for posterity sake. Going to take the truck for a drive now to get her to operating temp
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Old 05-22-2021, 11:39 PM   #4
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Ok, I blew through a small can of starter fluid with no luck. Engine was totally warm, doing the RPM drop and all. I never heard it surge once and I was going after every vac hose in sight.
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Old 05-23-2021, 12:07 AM   #5
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To add to the troubleshooting...you can loosen & then retighten the throttle body bolts(6mm Allen head 3/8Ē Dr. socket works best), exhaust manifold nuts(12mm) to the cylinder head, exhaust manifold to midpipe nuts(3 of them; 13mm) & check the midpipe crush washers for splitting(they are circular & often overlooked).

If itís only happening when the engine is at operating temperature. Then it appears that whatever has the leak. Itís happening while the component like a hose fitting, cylinder head, exhaust manifold etc. has expanded with the heat.
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Old 05-23-2021, 12:36 AM   #6
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Thanks for the input. I recently replaced the exhaust manifold put new gaskets on and I know I have a small leak between the exhaust manifold and the midpipe that I can't for the life of me eliminate. I even re-torqued the exhaust manifold to the cylinder, separated the midpipe from the manifold, checked the crush washers shot some high temp copper exhaust gasket maker in there and it still leaks very slightly.
How would the exhaust leak cause the vacuum leak though? Honest question. Not trying to be a smart ass.
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Old 05-23-2021, 12:44 AM   #7
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it wont. the aft side of the head has 0 to do with the intake side or vacuum leak. If you hose down the intake manifold where it meets the head and the throttle body gasket and theres no change in idle and you are SURE theres a vac leak, its gott be the hoses.

I just replaced all the hose i could reach for 25 bucks. its all 5/32 for the small shit. it cost me like 89 cents a foot at vatozone and i got 25ft. i think 25 is perfect. i missed like 6 hoses cause i couldnt jam my hands in there and i have like 3ft left over. its worth the 2-hours outside doing the job for so many reasons.
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Old 05-23-2021, 12:59 AM   #8
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Right on brother. I already ordered 20ft of silicone hose off Amazon and I'll start replacing those old rubber hoses as far as I can reach them. Very least it'll make the engine bay look better, and at best it fixes the leak.
I am sure it's a vac leak because I popped off the brake booster line, plugged the pressure/vac gage into it and watched it go from 23 down to 18 at the exact moment the RPM dropped, then watched it recover.
Any consideration to replace the larger hoses like brake booster, pcv to filter housing? I don't even know what size those big ones are.
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Old 05-23-2021, 11:47 AM   #9
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Quote:
Originally Posted by Rebellion32 View Post
Right on brother. I already ordered 20ft of silicone hose off Amazon and I'll start replacing those old rubber hoses as far as I can reach them. Very least it'll make the engine bay look better, and at best it fixes the leak.
I am sure it's a vac leak because I popped off the brake booster line, plugged the pressure/vac gage into it and watched it go from 23 down to 18 at the exact moment the RPM dropped, then watched it recover.
Any consideration to replace the larger hoses like brake booster, pcv to filter housing? I don't even know what size those big ones are.
Take off one certain size hose, ONE at a time. Take it to the parts store and match it up with hose from their stock rolls. Buy as much hose as you think you will need to replace that size. Buy modern chrome clamps. Don't pull more than one hose at a time, or one size at a time or you will forget where stuff goes. Don't overtighten any hoses that go to aluminum fittings, like your water hoses. You can crack the fittings.

Trust me on this one.
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Old 05-23-2021, 10:24 PM   #10
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Quote:
Originally Posted by Rebellion32 View Post
Right on brother. I already ordered 20ft of silicone hose off Amazon and I'll start replacing those old rubber hoses as far as I can reach them. Very least it'll make the engine bay look better, and at best it fixes the leak.
I am sure it's a vac leak because I popped off the brake booster line, plugged the pressure/vac gage into it and watched it go from 23 down to 18 at the exact moment the RPM dropped, then watched it recover.
Any consideration to replace the larger hoses like brake booster, pcv to filter housing? I don't even know what size those big ones are.



i would if its cheap enough. but i personally didnt. I got lazy and didnt bother. plus they are so easy to reach ill do it when its not blazing hot out.
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