'78 Bus Runs Rough, Possible Vacuum Leak

[Amskeptic]

I'd like an expert read on those spark plugs (Colin?). They actually look perfect to me, except for the one on the far left which appears a bit rich.

Don't forget about the injector seals!

And back to your original comments, I think ALL the FI engines sound like they have a vacuum leak at idle. I think it's just the air whistling through the idle circuit.

For an air-cooled engine, the three plugs to the right are at the border of lean. Did these plugs come out after a bunch of local driving or fresh off the freeway? I think the reason that the left plug (#1?) looks a little rich (actually pretty good) is because that cylinder is not working as hard with a 90# compression reading. Your clear sign of danger is when the outer ring has a white dusting and if the ground electrode looks blasted clean with orange-ish metal and white powder.
Colin

[dtrumbo]

Don't forget about the injector seals!

Those were replaced 5000 miles ago along with the fuel lines and filter. After I changed them I enjoyed a noticeable difference in engine power and overall engine smoothness. Yes, they do make a difference!

Did these plugs come out after a bunch of local driving or fresh off the freeway?

The plug photo is after a 60-70 mph freeway commute lasting about 30 minutes followed by a 35-40 mph city-street drive for about 15 minutes from the freeway to my house followed the next day by a ten minute idle with occasional 2000 RPM rise to check the mixture. My learning eyes agree that plugs #2-#4 are indicating a slight-lean condition with #1 looking pretty good. I will go out later this morning and put my vacuum gauge on and see what it reads and how stable it is.

[dtrumbo]

I hooked up my vacuum gauge to the input of the decel valve by using a 'tee' and a short piece of hose. At idle I measure -10 Hg with steady pulsing to -9 Hg (from what I've read, this is normal). At around 2000 RPM, I measure -15 Hg solid with no pulsing. If I let go of the throttle, the gauge blips higher and then settles to -10 where it was before. I pulled the hose off the decel valve, plugged it and the measurements stayed the same so I'm pretty sure my decel valve is good. I then moved my gauge upstream to the tap directly off the intake air distributor (plenum). Same readings, all appear to me to be correct or at least acceptable. Taking a cue from Don, I used a piece of vinyl tubing as a stethoscope and tried to find the source of my assumed vacuum leak. I listened to all of the intake boots, at the intake manifold gaskets and at all the other vacuum hose connections. The ONLY place I could hear air sucking was from underneath the plenum between it and the engine case. I've reported this in past topics and I seem to recall the consensus was that this is normal.

I offer this for your approval. Assuming the sucking sound from under the plenum is normal, is it possible that I really don't have a vacuum leak (or at least not a major one) and perhaps somehow the mixture on my bus has changed. Saying it out loud begs the question of how the heck did the mixture change? Remember, spraying Gumout on all the intake boots, at the intake manifold gaskets and even some willy-nilly under the plenum has not increased engine speed in any discernible way. With a known lean condition, I would think if there was a legitimate vacuum leak, it would suck some Gumout in and I would hear the RPM tick up at least a little.

May I adjust my AFM to achieve the correct mixture and go on about my day? I know I will need to rebuild this engine within the next 5000 miles or so, given #1 cylinder's slow deterioration at which time I, of course, will replace all the boots and gaskets, etc. I'm not trying to weasel my way out of doing this work (o.k. maybe just a little), but I really don't want to go through the hassle of replacing all that stuff and not fix my problem.

[Amskeptic]

vacuum -10 Hg with steady pulsing to -9 Hg
2000 RPM -15 Hg solid
appear to me to be correct.

Assuming the sucking sound from under the plenum is normal, is it possible that I really don't have a vacuum leak (or at least not a major one) begs the question of how the heck did the mixture change?

May I adjust my AFM to achieve the correct mixture and go on about my day?

Those are low readings for FI.

You may have valve guide wear, which leans the mixture. You have checked oil fill plastic-to-pipe interface and dipstick seal?

By all means have at the AFM. Remember, it is not the PC Armchair Engineer Weenies such as myself who matter here. What matters is what the engine tells you. If it tells you it is happy, it is happy. Get that mixture to a slight lean at idle and a slight lean at 3,000 rpm, and if the power and driveability is good, yer good.
Colin

[dtrumbo]

You have checked oil fill plastic-to-pipe interface and dipstick seal?

My 'taco plate' is leaking which I believe is the cause of the oil all around the area of the aforementioned filler pipe-to-case and dipstick area. That said I ordered a new gasket for the filler pipe along with the new taco plate gasket from Scott. I also have a new dipstick boot as well. I'll install the gaskets and attempt to install the dipstick boot (It appears to be a royal PITA) at the next oil change which I will do four hundred miles early to help rule out these items contributing to my vacuum leak.

By all means have at the AFM. Remember, it is not the PC Armchair Engineer Weenies such as myself who matter here. What matters is what the engine tells you. If it tells you it is happy, it is happy. Get that mixture to a slight lean at idle and a slight lean at 3,000 rpm, and if the power and driveability is good, yer good.
Colin

Understood. What I've taken away from this discussion as well as other related topics on this forum is "what makes your exhaust valves happy, makes YOU happy!" Would I like to find and fix my vacuum leak? Absolutely! Until then, I'm going to make my valves happy by getting the mixture correct for the current conditions so I can drive it with some level of confidence that the day my rebuild becomes necessary won't arrive prematurely.

[Amskeptic]

You have checked dipstick seal?

I have a new dipstick boot.

Dipstick seal is a little cork deal under the "hat" that contacts the metal dipstick tube that goes through the fan housing as you push the dipstick down. It is not responsible for lean running but it contributes. As importantly, it keeps nasty dust abrasive contaminates from sucking down in there.
Colin

[dtrumbo]

Is there always a certain amount of vacuum present in the crankcase or is this an indication of wear which over time creates vacuum in what should otherwise be a 'neutral zone'? I'm still getting my brain wrapped around why the idle drops when I take the oil filler cap off.

[Amskeptic]

Is there always a certain amount of vacuum present in the crankcase or is this an indication of wear which over time creates vacuum in what should otherwise be a 'neutral zone'? I'm still getting my brain wrapped around why the idle drops when I take the oil filler cap off.

The breather hose coming from the S-boot does normally have a little negative pressure, this makes sure that vapors get *drawn* into the intake airflow for burning. You should expect then, that taking off the oil filler cap would result in a lean drop. A well-worn engine under load will happily provide positive pressure up through that hose to the S-boot. It will also happily attempt to expel air and oil vapor from the dipstick, the crank seals, the valve covers. A fresh engine can remain clean indefinitely with that slight negative pressure through that side of the "circuit". The other side of the circuit is the air coming in through the intake, passing through the valve guides and the piston rings to the crankcase. It is a circular path of course. The more intake air that passes through the guides and rings as they wear adds more flow to the breather, but note that it enters the engine under the AFM so it is robbing the engine of metered air. This gives rise to the lean-creep over the life of the engine that requires monkeying with the AFM.
Colin

[dtrumbo]

Alrighty then. The oil has been changed and while it was drained I changed the o-ring around the taco plate. I couldn't find a torque value for the taco plate bolts so I went easy on them with 5 ft./lbs. (60 in./lbs.) and so far no leaks. If anyone knows the correct torque value, please let me know. I attempted to change the oil filler pipe gasket, but couldn't remove the filler pipe rearward enough to clear the studs. The engine hanger bar was in the way so I didn't push it. I was successful at replacing the hardened-beyond-brittle dipstick bellows which caused me to perform the happy dance at 6:15am in my 50 degree shop. This is the second-hardest thing I've done to my bus(es). First place still belongs to reattaching the horn wire to the spade connector behind the steering coupler. New oil, new filter, fired her up, no leaks.

So now back to the original issue. I drove the bus for a nice twenty-minute warm up, shut her down and then moved the wiper on the AFM counter-clockwise a bit. This requires you to bend the little bar that actuates the switch a bit so the fuel pump will shut off properly. Then I fired her back up and now my idle was way too high from where I had monkeyed with it earlier. I turned that back down where it belonged and nudged the wiper again. It was A LOT better, but it needed to go a little more. I moved it a little more and then 'fined tuned it' by finding my peak idle RPM using the AFM bypass "trim" screw. I now am in the middle-to-rich side at both idle and 2k RPM. For sport, I checked my timing and with the advance hose pulled off the distributor and plugged I'm spot on 28 degrees BTDC. I put the hose back on and it went up to 40 at high RPM. At idle it's right at 0 degrees instead of the ideal 7.5 BTDC. I hope this is correct. Again, chime in if anything isn't.

Ah... that's better! Thank you!

I drove the bus to work today and WOW does it run nicer! Smooth idle, smooth acceleration, 75 MPH no problem. I know I have now covered up a problem by treating the symptom rather than the cause, but at least I won't be burning up my exhaust valves by driving it until I tear this beast down someday and do a proper rebuild which will certainly flush out this damn vacuum leak. I'll keep driving it back and forth to work occasionally this week to keep an eye on things before the big trip to Nehalem Bay Super Bowl weekend.

Thanks everyone for all your help.

[Amskeptic]

Alrighty then.
I drove the bus to work today and WOW does it run nicer! Smooth idle, smooth acceleration, 75 MPH no problem. I know I have now covered up a problem by treating the symptom rather than the cause, but at least I won't be burning up my exhaust valves by driving it until I tear this beast down someday and do a proper rebuild which will certainly flush out this damn vacuum leak.

You might have an actual cure here. Valve guide wear can cause a mystery lean condition.
I follow the If The Engine Is Happy, I Am Happy method of maintenance. This allowed me to get, for example, 515,000 miles on my original cylinder heads regardless of professional opinions that threatened me with dire failure. Just keep an eue on thiongs, but if it is peppy and running cool and not eating oil . . . yer good.
Colin

[dtrumbo]

You might have an actual cure here... but if it is peppy and running cool and not eating oil . . . yer good.
Colin

No cure. Just different. Now it's hard to start and runs like crap when it's cold. A nudge of the wiper reveals it's now pig-rich when cold, but after it warms up it smooths out (this is a VERY relative statement) and the mixture seems to be correct. Pulling the plug wires one at a time reveals cylinder #1 has started summer vacation early. At idle, there is hardly any discernible change with the plug wire on or off.

It does run cool and uses no oil which I'll have to be content with for now. After the big trip next weekend, I'll do a leakdown test. What's that you scream? WHY THE HELL HAVEN'T YOU DONE THIS BEFORE NOW??? O.k. o.k. I'm lame. I've had the tester for over a year and never had the courage to teach myself how to use it, largely because my vehicles are (were) in relatively good shape. Well now this bus is in the condition where this handy-dandy little tester might offer a nice preview of what I'll find when I tear this bad boy down this fall (I hope it makes it that long!).

I'll be back.

[Amskeptic]

cylinder #1 has started summer vacation early. At idle, there is hardly any discernible change with the plug wire on or off.[/quote]

Cold or warm and cold? Was there a compression issue with this cylinder?
Indeed, you are now out of the envelope when cold.

It does run cool and uses no oil which I'll have to be content with for now.

Can you lean it out a little, to help cold running? You have some wiggle room here with cool running when warmed-up (and if you have decent power). At warm engine high speed check, does rpm remain mostly steady when you move the wiper CCW? If so, lean it down with the spring until you get a slight uptick in rpm when you move the wiper CCW. Just a bit. Then check plug colors. If compression is bad on #1, leave it all be, yer good for the circumstance.
Colin

[dtrumbo]

If compression is bad on #1, leave it all be, yer good for the circumstance.
Colin

#1 is 95 compared to 130, 130, 140 for the others. I'm leavin' it be.

[dtrumbo]

After the big trip next weekend, I'll do a leakdown test...
I'll be back.

Well alrighty then. Next weekend has turned into six months later and I finally did the leakdown test! No surprises, which in a way is good. I got up early, drove the bus for twenty minutes to get it warmed up good, pulled the plugs (which I put anti-seize on last weekend in anticipation of removing them hot) and put the heretofore unused tester to use. The results...

#1 - 50% leakage. Air heard escaping from the oil filler (rings) AND with the throttle valve held open, the air filter intake horn. Sounds like #1 intake valve is having a difficult time sealing. Burned? Dropped seat? Both? Neither?

#2 - 37% leakage. Air heard escaping from the oil filler only.

#3 - 32% leakage. Air heard escaping from the oil filler only.

#4 - 30% leakage. Air heard escaping from the oil filler only.

This info jives with my compression test results of 95, 130, 130 and 140.

I intend to do an oil pressure test and also measure end-play to further evaluate the condition of this engine. I spoke briefly with Colin at Maupin about considering just reconditioning the heads if the other engine-health parameters seem o.k. This course of action, especially with the current shortage of bearings, seems like a plan to at least be considered.

Thoughts?

[dtrumbo]

O.k. I've ordered the adapter to use my new oil pressure gauge with the 1/8-27 fitting in the M10-1.00 sender hole. I'll have to wait patiently for the mail carrier.

In addition, I bought a dial indicator and the magnetic holder gizzie so I can measure my end play. I seem to recall reading that you can measure end play with the engine in the bus by simply pushing (with all your might) on the fan hub, set your dial indicator and then push the clutch in. Did I dream this? I even seem to think it was Colin that wrote that, but I can't swear to it and hope he won't sue me for slander. Author aside, does this sound like a reasonable way to get a ballpark end play measurement?