318 Internal Vacuum Leak?

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govolsbeatbama

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Hey everyone, I read on this forum a lot but finally made a profile so I could try and figure out this issue. I have a 318 from a 1990 W150 and I put a holley sniper 2 on it with an old streetmaster intake. When I start it up it wants to idle around 1500 or so and I can only lower it around 1000 until the idle screw can't be turned out anymore. The goal was to get it to 750 at idle. I've unplugged and capped all vacuum lines to rule out that, sprayed cleaner around and did a smoke test which it passed. Holley had me send the unit back to them to test it, but on the phone they said they've never heard of a sniper 2 having this happen and thought it might be an internal vacuum leak. I've read that you can test this, so I hooked up a vacuum gauge to the dipstick and plugged the crankcase pressure so it only reads out of the dipstick. It read that there was pressure which according to some other threads meant no leak. I'm stumped on this and everyone I ask says there's a leak somewhere but I think all these tests eliminated that as a possibility. As a broke college kid I've put way to much money into trying to figure this out. Any suggestions?
 
Have you verified that all vacuum hoses are sealing properly?

And if it is internal, it’s not hard to just pull the intake and reseal.
 
Make sure the throttle cable isn't holding it open. Easy check, just remove the linkage from the TB.
 
govolsbeatbama said:
Hey everyone, I read on this forum a lot but finally made a profile so I could try and figure out this issue. I have a 318 from a 1990 W150 and I put a holley sniper 2 on it with an old streetmaster intake. When I start it up it wants to idle around 1500 or so and I can only lower it around 1000 until the idle screw can't be turned out anymore. The goal was to get it to 750 at idle. I've unplugged and capped all vacuum lines to rule out that, sprayed cleaner around and did a smoke test which it passed. Holley had me send the unit back to them to test it, but on the phone they said they've never heard of a sniper 2 having this happen and thought it might be an internal vacuum leak. I've read that you can test this, so I hooked up a vacuum gauge to the dipstick and plugged the crankcase pressure so it only reads out of the dipstick. It read that there was pressure which according to some other threads meant no leak. I'm stumped on this and everyone I ask says there's a leak somewhere but I think all these tests eliminated that as a possibility. As a broke college kid I've put way to much money into trying to figure this out. Any suggestions?
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Welcome to the forum!!!
 
Have you put a vacuum gage on it. Whats the timing set at?
 
remytherat said:
Have you verified that all vacuum hoses are sealing properly?

And if it is internal, it’s not hard to just pull the intake and reseal.
Click to expand...
I checked and wiggled each vacuum hose on the ports which tells me they're sealing properly. I agree its not necessarily hard to reseal the intake and I'm thinking I'll do that if it comes back that the sniper was fine, I just didn't want to go spend more money on another set of gaskets, pull everything apart and put it back together just to have it happen again. This was my first intake swap, any tips?
 
OldmanRick said:
Make sure the throttle cable isn't holding it open. Easy check, just remove the linkage from the TB.
Click to expand...
I didn't disconnect the cable, but I pulled it forward so there was slack and pushed on the throttle to make sure it was closed and it didn't change anything. When I get it back though I'll get it set up without the cable attached to see if that was the problem.
 
I had an intake manifold leak, couldn’t find it but assumed it was leaking in the valley. I installed a different manifold, with new Fel-Pro gaskets and eliminated the cork end seals. Your power brake booster could be leaking also
 
The throttle screw is generally used to adjust the IAC (inlet air control) Idle speed is adjusted in the App
Might be worth looking at some YouTube videos?


 
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As to a vacuum leak;
You said it passed all the external leak tests AND, that you have pressure in the CC, so
IMO, if you did a good job, then you can rule out a vacuum leak.

By backing out the speed screw, the throttle valves are fully closed. therefor, air is entering the intake from somewhere. The most likely sources are the booster hose or the PCV system.
BUT
If your TB has an Automatic Idle Speed Control, there is a good chance that it is open just way too far. That device has to be synced to the minimum throttle opening to tell the ECU about how much air the engine is getting. @Cal Tonsley mentioned this, above.

BTW
govolsbeatbama said:
. As a broke college kid I've Any suggestions?
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Since you're in the HOBBY, and already have a Sniper system, at such a young age, You'll not get much if any sympathy from me. lol........ I'm 71, and I've been down that road more than a few times. I run a carb. There is a good reason for that.. Me and carbs are good friends, and once set up, just about nothing ever goes wrong with them, and if you had one, you wouldn't have
"put way to much money into trying to figure this out."......... lol.

Ok, I'm done yanking your chain. I'm sure you'll have it running like a champ real soon.
 
So here's an update, I ended up pulling the intake and resealed it with the gaskets under warranty. Started it up again with all vacuum ports on the TB plugged and still had the issue. After I put it back on I didn't reconnect the linkage to eliminate that as the cause. Spraying cleaner at the china walls and sides of the intake made no difference. Before I did this, Holley wanted to test the unit but it passed all the tests on their end as well as completely closing the IAC so that's not the problem. I pulled the spark plugs and attached a pic but one or two were loose enough to be taken out just by hand. Could that cause a high idle at all?
20240714_185901.jpg

20240714_185907.jpg

My neighbor suggested I do a compression and leakdown test to make sure the valves were ok so I'll probably do that later today just because as well as another smoke test. Would anything like a cracked valve or something cause a high idle? I'm really at my wits end with this because it's been down since December and I feel like every test comes back fine but the problem still stays.
 
I'll go out on a limb and say that your valves and rings are fine enough to not be the cause of your problems. Your 10 degrees of Idle-timing is fine.
I'm going with the ECU not knowing the Engine temp, or the temp is not getting high enough to trigger the ECU.

With EFI, (as with a carb) assuming the throttles can be closed;
there are only a few things that can cause a High Idle, namely;
1) too much Idle-air, and
2) too much Idle-Timing
3) a faulty warm up program/ECU stuck in open loop/AIS not coming down.

IMO, to set the base settings, one needs to defeat both the Closed-loop system, and the Automatic Idle-Speed motor; which means taking the 02s off line. As long as the ECU knows about the issue, it's just gonna keep adjusting the AFR to get to whatever it has been programmed to do.
As for numbers 1 and 2 above, you probably got that covered.
As for #3, above; Your EFI must have a warm up program. Which is controlled by either Coolant temperature or time, or some combination of those. During this time, the ECU is already in open loop. Perhaps your coolant temp sensor is not working? and the ECU is waiting for the temp to come up. During this time, the ECU is on a base map, but the AIS controlling the Idle speed to a programmed rpm.
>Disconnecting the AIS will not affect the idle speed, once the AIS has jacked it up.
>After you shut the engine off, the ECU is pre-programmed to adjust the AIS to a high idle, so that, on the next cold start, the AIS will already be at a high idle.
>After the ECU figures out that the engine is warming up, it will reduce rpm via the AIS, proportionally to what she is seeing at the CTS.
>If you start it up hot, the ECU is programmed to rapidly bring the rpm down, proportionally, to what the CTS is telling the ECU.
>At the moment when you shut the engine off, the ECU is programmed to shut the air off at the AIS, to prevent run-on. This means that the Curb-Idle Screw has to be set closed enough, to not interfere.

All of this assumes that the ECU knows what the Coolant Temp is. Therefore, I would prove that device, and it's circuit, and/or if you can electronically get into the Sniper ECU, like with a scanner, I would check to see what the ECU is reading.
Good luck
 
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3) a faulty warm up program/ECU stuck in open loop/AIS not coming down.

IMO, to set the base settings, one needs to defeat both the Closed-loop system, and the Automatic Idle-Speed motor; which means taking the 02s off line. As long as the ECU knows about the issue, it's just gonna keep adjusting the AFR to get to whatever it has been programmed to do.
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This is exactly what I needed! I read this a few times and got a weird idea to do a reset from the beginning. Turns out it must have been a bad initial tune where the computer learned some bad habits and stuck with them since I didn't turn off timing control on first startup but only later on. I ended up turning off closed loop on the handheld, reloaded the initial wizard and started it up. Adjusted the idle screws a little bit and got it around 750 with the IAC around 2-3%. Turned closed loop back on, went through the different gears and revved a little and it always went back to where it should be. Thank you so much for your suggestions, I would've never thought about this even being a possibility!
 
How much vacuum does the engine have at idle?
 
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