Need some engine theory

[delco1946]

Time to renew my truck again . I unsurprisingly failed emissions. I have been fiddling with the idle air mixture screws and I’ve called it a night. It doesnt idle well under 4 screw turns ( lopsided idle), but it’s failing from running too rich.

I plan to verify that my EGR is not blocked open tomorrow with crud contributing to poor idle.

I do have a question regarding the PCV system, just replaced the valve, and it seems to be sucking in air at idle. My understanding was that it should not be sucking any amount of crank case emissions in at idle as that makes idle rougher. Can someone confirm or deny this? It seems to be directly “wired” to manifold vacuum, don’t think I can control it much other than by vacuum ( so then does that mean my vacuum is somehow off at idle )??Thanks!

[Gary Lewis]

The PCV valve is a "controlled leak". It is always going to leak some.

But if you have to have the idle air mix screws open more than 4 turns then I think you have a vacuum leak - other than the PCV. I'd expect the screws to be in the 1 1/2 to 2 1/2 turns open range. So look for a split vacuum line or some other vacuum leak.

But the EGR valve could be part/all of the problem, so do check it out. If it is leaking it'll act like a vacuum leak.

[delco1946]

The PCV valve is a "controlled leak". It is always going to leak some.

But if you have to have the idle air mix screws open more than 4 turns then I think you have a vacuum leak - other than the PCV. I'd expect the screws to be in the 1 1/2 to 2 1/2 turns open range. So look for a split vacuum line or some other vacuum leak.

But the EGR valve could be part/all of the problem, so do check it out. If it is leaking it'll act like a vacuum leak.

Thanks Gary, and to clarify, it does run at idle starting at around 2 1/2 screw turns but it definitely isn’t idling well, and it doesn’t seem to matter if I feather the gas as it still sounds like there’s a vacuum leak.

However, I have already replaced all the vacuum hoses so it shouldn’t be because of split rubber or anything, as straightforward as that unfortunately but I’ll take a look in the daylight tomorrow to see what I can find. The EGR valve has also been replaced under my watch, but I’ve read multiple accounts where crud in the engine got sucked up into it, and somehow got it stuck open resulting in a “vacuum leak”.

[grumpin]

Thanks Gary, and to clarify, it does run at idle starting at around 2 1/2 screw turns but it definitely isn’t idling well, and it doesn’t seem to matter if I feather the gas as it still sounds like there’s a vacuum leak.

However, I have already replaced all the vacuum hoses so it shouldn’t be because of split rubber or anything, as straightforward as that unfortunately but I’ll take a look in the daylight tomorrow to see what I can find. The EGR valve has also been replaced under my watch, but I’ve read multiple accounts where crud in the engine got sucked up into it, and somehow got it stuck open resulting in a “vacuum leak”.

Check the vacuum hoses at the firewall junction, particularly the one going to your C6.

Someone here found theirs swollen from tranny fluid, I noticed my old truck had that problem.

Pulled the hose at the tranny and fluid came out. Had to replace the vacuum modulator and hoses.

[delco1946]

Check the vacuum hoses at the firewall junction, particularly the one going to your C6.

Someone here found theirs swollen from tranny fluid, I noticed my old truck had that problem.

Pulled the hose at the tranny and fluid came out. Had to replace the vacuum modulator and hoses.

Didn’t even know / think of a vacuum hose going to the tranny. Is this on the drivers or passenger side? I guess I’ve never seen this. Thanks for the tip.

Also was reading about the rag trick to better assess if less air at the carb makes it idle better.

[Gsmblue]

Didn’t even know / think of a vacuum hose going to the tranny. Is this on the drivers or passenger side? I guess I’ve never seen this. Thanks for the tip.

Also was reading about the rag trick to better assess if less air at the carb makes it idle better.

Passenger side near the top of the firewall.

If not there, follow the lines from the transmission.

[delco1946]

Passenger side near the top of the firewall.

If not there, follow the lines from the transmission.

So I did find a hose that had popped off from the intake manifold, reinstalled that. I checked the rubber hose from the vacuum splitter at the firewall to the transmission it’s a short piece of hose and look fine I also checked the hose to the brake booster which has been rebuilt in the last four years so I would expect it to be OK.

I have the air fuel mixture screws down to around 2.5 turns it can go a little lower but it still doesn’t idle well at that, and if I hold a gloved hand over the carb intake the engine rpm picks up and it runs smoother which does seem to indicate that there’s a vacuum leak somewhere. I have yet to remove the EGR to verify that it’s clean internally, it’s been years since I’ve done that so doing a bit of research before attempting .

I took a hose stethoscope and poked it all over the place and don’t hear any leaks other than a lot of hissing coming from the top inside of the carb but I assume that’s normal? Carb has been rebuilt with new spacers.

Looking for other suggestions to check. I did take pliers and pinched all the hoses I could find and have yet to come up with anything that improve the engine performance other than blocking off the carb intake with a glove.

[delco1946]

So I did find a hose that had popped off from the intake manifold, reinstalled that. I checked the rubber hose from the vacuum splitter at the firewall to the transmission it’s a short piece of hose and look fine I also checked the hose to the brake booster which has been rebuilt in the last four years so I would expect it to be OK.

I have the air fuel mixture screws down to around 2.5 turns it can go a little lower but it still doesn’t idle well at that, and if I hold a gloved hand over the carb intake the engine rpm picks up and it runs smoother which does seem to indicate that there’s a vacuum leak somewhere. I have yet to remove the EGR to verify that it’s clean internally, it’s been years since I’ve done that so doing a bit of research before attempting .

I took a hose stethoscope and poked it all over the place and don’t hear any leaks other than a lot of hissing coming from the top inside of the carb but I assume that’s normal? Carb has been rebuilt with new spacers.

Looking for other suggestions to check. I did take pliers and pinched all the hoses I could find and have yet to come up with anything that improve the engine performance other than blocking off the carb intake with a glove.

I removed the EGR valve and checked it and also applied vacuum to it with the engine idling and it almost killed the engine so both test indicate the valve works fine. As I mentioned previously, I have pinched off hoses without noticing a change. Not sure where else to check for a vacuum leak - I didn’t hear any hissing at the base of the carb in case the gasket had torn, etc.

[Gary Lewis]

I removed the EGR valve and checked it and also applied vacuum to it with the engine idling and it almost killed the engine so both test indicate the valve works fine. As I mentioned previously, I have pinched off hoses without noticing a change. Not sure where else to check for a vacuum leak - I didn’t hear any hissing at the base of the carb in case the gasket had torn, etc.

David/1986F150Six had a similar problem and just could NOT find a vacuum leak. He tried squirting brake cleaner everywhere, to no avail. Finally he tried the smoke test and immediately found the problem - a vacuum cap had apparently split - in all senses of the word.

What he did was to get a cheap cigar, block off the inlet to the carb, and puff smoke into a large vacuum line - either the PCV line or the brake booster hose. That put smoke throughout the engine and magically a wisp of smoke came out from under the intake manifold on that 300. Upon close inspection he found a port was open way back in the depths where he couldn't see nor get brake cleaner to.

[ratdude747]

David/1986F150Six had a similar problem and just could NOT find a vacuum leak. He tried squirting brake cleaner everywhere, to no avail. Finally he tried the smoke test and immediately found the problem - a vacuum cap had apparently split - in all senses of the word.

What he did was to get a cheap cigar, block off the inlet to the carb, and puff smoke into a large vacuum line - either the PCV line or the brake booster hose. That put smoke throughout the engine and magically a wisp of smoke came out from under the intake manifold on that 300. Upon close inspection he found a port was open way back in the depths where he couldn't see nor get brake cleaner to.

Ditto. Happened to me twice on my 300... and both times I caught it with a party fogger converted to a cheap smoke machine (I don't smoke, and have no intention to start!). The whole top of the cap was gone in both cases, resulting in a very rough/unstable idle.

If the caps are old, I'd replace all of them as insurance.