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Need some engine theory

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delco1946

Need some engine theory

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!

Christopher in Portland Or. 1981 F250 Ranger XLT 4x4 trailer special 351M, and auto C6

Gary Lewis

Re: Need some engine theory

Administrator

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.

Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI

delco1946

Re: Need some engine theory

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”.

Christopher in Portland Or. 1981 F250 Ranger XLT 4x4 trailer special 351M, and auto C6

grumpin

Re: Need some engine theory

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.

Dane
1986 F250HD SC XLT Lariat 4x4 460 C6-Sold
1992 Bronco XLT 4x4 351W E4OD
1998 GMC Sierra SLE K1500 350 4L60E
Arizona

delco1946

Re: Need some engine theory

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.

Christopher in Portland Or. 1981 F250 Ranger XLT 4x4 trailer special 351M, and auto C6

Gsmblue

Re: Need some engine theory

Passenger side near the top of the firewall.

If not there, follow the lines from the transmission.

1985.5 F-150 XL Explorer standard cab 5.0 EFI AOD 4x4
Daily Driver. We call her Eunice the Ute.

1982 Bronco XLT Lariat 351W AOD 4x4
Code name Esperanza, or Espy to her friends. Please see my Project thread for the blow by blow.

1984 F-350 XL Centurion crew cab 460 T19 4x4
"Eylza Dual-little"

delco1946

Re: Need some engine theory

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.

Christopher in Portland Or. 1981 F250 Ranger XLT 4x4 trailer special 351M, and auto C6

delco1946

Re: Need some engine theory

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.

Christopher in Portland Or. 1981 F250 Ranger XLT 4x4 trailer special 351M, and auto C6

Gary Lewis

Re: Need some engine theory

Administrator

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.

Gary, AKA "Gary fellow": Profile

ratdude747

Re: Need some engine theory

Gary Lewis wrote

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.

1984 F150: 300 L6, AOD, RWD. EEC IV / TFI, Feedback Carter YFA Carb. Stock everything but radio (for now).

mat in tn

Re: Need some engine theory

go the extra inch and get a pkg. of vinyl vac caps. they do not degrade the way rubber does. these are the pretty bright color coded ones.

delco1946

Re: Need some engine theory

Tried smoking out my engine and couldn’t find any leaks :/. Not sure what to do next.

Christopher in Portland Or. 1981 F250 Ranger XLT 4x4 trailer special 351M, and auto C6

FuzzFace2

Re: Need some engine theory

In reply to this post by ratdude747

ratdude747 wrote

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.

Could you post up in the garage / tool area how you did the party fogger to smoke machine.

I have a car club member that has a vacuum leak that I think is between the intake & heads (v8) as we have not found it in the quick checking 2 of us have done so far.
I want to fill the crank case with smoke and see if it will come out the carb.

It started with the idle speed going to about 1500.
Told he changed carbs and it is still doing it.
I need to get some time to get with him to really look it over because it was only a min or 2 before going to a show the last time I looked it over.
Dave ----

Dave G.
81 F100 flare side 300 six / AA OD / NP435 / 2.75 gear
http://cars.grantskingdom1.com/index.php/1980-Ford-F100?page=1
81 F100 style side 300 six/SROD parts truck -RIP
http://cars.grantskingdom1.com/index.php/1981-Ford-F100

Gary Lewis

Re: Need some engine theory

Administrator

In reply to this post by delco1946

delco1946 wrote

Tried smoking out my engine and couldn’t find any leaks :/. Not sure what to do next.

So the problem you are now having is that the idle isn't smooth but it is idling best with the screws 1 1/2 turns out? Right?

Gary, AKA "Gary fellow": Profile

85lebaront2

Re: Need some engine theory

Administrator

I found years ago that the 351M and 400 engines were a problem, Ford actually got fined over them by the EPA. In order to idle smoothly the mixture has to be quite rich. For those of us who did not have emission inspections, opening up the idle mixture tubes in the cluster worked wonders.

Bill AKA "LOBO" Profile

"Getting old is inevitable, growing up is optional" Darth Vader 1986 F350 460 converted to MAF/SEFI, E4OD 12X3 1/2 rear brakes, traction loc 3:55 gear, 160 amp 3G alternator Wife's 2011 Flex Limited Daily Driver 2009 Flex Limited with factory tow package Project car 1986 Chrysler LeBaron convertible 2.2L Turbo II, modified A413

Gary Lewis

Re: Need some engine theory

Administrator

Bill - You'd recommend opening up the idle screws more to smooth out the idle?

Gary, AKA "Gary fellow": Profile

delco1946

Re: Need some engine theory

In reply to this post by Gary Lewis

They’re currently about 2.75 turns out. However if I place a rag/ gloved hand over the carb it clearly idles better and smoother.

I think the goal would be to pass less air thru the carb rather than simply increase the gas thru the carb as that’s why I’m failing emission and getting poor mileage.

Also, I have noticed that the hissing/ sucking noise from the top of the carb/from inside the carb is quite loud. Is that normal?

Christopher in Portland Or. 1981 F250 Ranger XLT 4x4 trailer special 351M, and auto C6

delco1946

Re: Need some engine theory

In reply to this post by FuzzFace2

I held 5 small cigars to one end of my shop vac and held the blower side hose up to and taped to the brake booster line. The cigars burn fast but it def blow a lot of smoke out the top of the carb at the rag!

Christopher in Portland Or. 1981 F250 Ranger XLT 4x4 trailer special 351M, and auto C6

85lebaront2

Re: Need some engine theory

Administrator

In reply to this post by Gary Lewis

He's between a rock and a hard place with that engine and emission testing. I would rather try to get a hot cammed eye burning exhaust small block Chevy through.

If he opens the screws he will be richer. One thought, fresh plugs gapped a bit wide, oil change, new PCV valve and set it to factory mixture. This is what some of my friends in California do before their tests.

A wider plug gap will fire a leaner mixture reliably.

Bill AKA "LOBO" Profile

FuzzFace2

Re: Need some engine theory

In reply to this post by delco1946

delco1946 wrote

I held 5 small cigars to one end of my shop vac and held the blower side hose up to and taped to the brake booster line. The cigars burn fast but it def blow a lot of smoke out the top of the carb at the rag!

Thanks for that
I Googled on how to make one also.
Dont know if I want to take up smoking (again) so may make one.
Thanks again
Dave ----