Erratic Vacuum Readings and Rough Idle

[Steve73QMach1]

I am having a problem with my Mach 1. The engine was just rebuilt but the car will not idle smoothly. The engine is the original 351C-4V but has an Edelbrock Performer intake and carb installed. It has the C6 transmission. Vacuum at idle is about 14" and fluctuating +/- 2" at idle. At 2,000 RPM (no load) vacuum is steady at about 18". At 3000 RPM it is at 20" and steady. This seems backwards. With the air cleaner off, the passenger side valve cover vent is streaming out a fog of what smells like air/fuel mixture or exhaust. The car will idle but only at 1000 RPM. Even then, after 5 minutes, it loads up and dies unless I feather the throttle. Have not checked timing or performed a compression check yet. I will do that tomorrow. I have had very little time off work recently but have tomorrow off and would like to make the most of my time. The car is driveable to a degree in this condition but it is way down on power. As a result, it stays in the garage. It is not a fun vehicle to drive like this. I would appreciate any input to get me looking in the right place.

 

[72HCODE]

I'm thinking valve stem seal failure.

how long ago was the rebuild?

based on the fog coming out of the vent and the fluctuating vacuum I'm thinking issue with the valves.

pull the plugs inspect them, do a compression test seems like a good start. maybe pull the valve cover and inspect the rockers and springs.

has the engine been making oil dissappear at all?

any smoke on acceleration?

i had a situation where my engine builder used teflon seals originally. the teflon seals have a retainer ring after less then 500 miles i was losing oil and had fouled plugs and smoke on startup... i suspected the seals but i wasn't sure. my valve covers were leaking so i suspected that was my oil loss.. I bought new gaskets(thicker) and when i removed my drivers cover i decided to inspect the rockers etc.

What i found was pieces of the teflon seals stuck in my oil gallery... I basically freaked out.. after fighting with my engine builder for 6 months, he finally agreed to inspect the damage. we found more of the seals had failed then just one. there were multiple fragments in my engine. at that time Vitron seals had just come on the market and we changed to the vitron which use a spring retainer instead of just a metal clip retainer. my oil consumption and smoke and lower vac went away after.

I would go with your gutt on this and do the compression check also.

 

[Steve73QMach1]

Thanks for taking the time to respond 72H! I appreciate your input. To answer some of your questions, the engine has about 50 miles on it since rebuild (heads rebuilt, block bored 0.030 over). I blows no blue smoke or black smoke while accelerating and does not appear to be using any oil - all good news. Compression test results indicated 120 psi (+/- 3) on all cylinders. Nearly all of the plugs were somewhat wet with a raw fuel odor. I also found that they had gapped the plugs to about 0.042 for some reason. I have always gapped the plugs to 0.035 in the past as it states on the valve cover sticker. I saw on one of your other posts that you are not a fan of the spiral core 8mm plug wires. That is what they put on my car (made by Accel). I also have a Pertronix III distributor and a Flamethrower coil. The coil is sending out a strong spark. I may have to go back to original style plug wires. Those 8mm are way too long to most cylinders.

 

[droptop73]

Possible intake manifold leak, carb, manifold gaskets, fittings, vacuum lines, etc.

 

[wwhite72]

from my personal experience i might suggest trying another coil. i just had to deal with a slightly similar situation . I had great spark at idle. didnt start to act badly until the revs climbed. and you say you have 50 miles on the rebuild? My coil started getting worse over time. Seems they can die from heat, or from vibration, or from the oil not covering all the windings when installed on their side...or if a mechanic leaves the key in the "on" position for a few minutes while he answers his cell phone. It is a cheap first step. and if there's no change you have a backup coil. And im sure your wires are fine. I have used MSD super conductors on 3 vehicles with great success. after that...well i guess you check where the rotor is pointed. But the guy who accepted your money should help, no?

 

[wwhite72]

Did the builder degree the cam? Sound like a valve issue but could be the timing chain is off a tooth as well.

 

[72HCODE]

all valid ideas to try,, the problem is now it becomes swap parts until you find the problem.

I'm no fan of the PetIII ignition system either, the Tach signal that comes off the PIII is a Square signal and it will not work with the factory tack. i've seen it do some weird things as well.

I went back to the PII because of the automatic dewell adjustment to help with startups.

I would agree the engine builder should be helping you, however my experience is those guys are usually scum after they get your money.

you can inspect the coil for obvious issues, for example I had a new MSD oil filled coil i put it and went for a spin, it didn't run right. when i came back i had oil on my intake that made no sense. turned out the oil inside the MSD coil designed to help keep it cool was leaking out through the top. the thing was basically defective out of the box.

could be a cam issue but the smoke is saying combustion gas is getting into the valve covers.

your plugs are fowling with fuel as well, that is saying ignition issue or a combo of unburnt fuel and oil fowling the plugs.

i would 100% regap the plugs to .35, then reset the initial timing, disconnect the vac accessories if you have any plug the vacuum connections and see what the vac readings are.

you could have a vac leak, but smoke coming out the valve covers would indicate combustion gas coming out of the valve seals.

ignition issues can mask as a valve issue. i suspect you have 2 issues, ignition and the valve stem seals or valves themselves.

you can run on bad valve stem seals what will happen is the plugs will get some oil fowling and you will consume oil as the stem seals get worse it will consume more oil and eventually the car will smoke at start up from oil getting into the cylinders.

What gets me +-2"HG flicker you see on the vacuum gauge.

check out this site

http://www.secondchancegarage.com/public/186.cfm don't go crazy if you see similar HG readings on your engine it is more of a guide to point you into diagnostics.

now there could be other options like poorly seated valves. that is possible since this is a new rebuild the machine shop could of screwed up the heads.

the valve springs could of also be incorrectly setup or tensioned that would require rocker adjustment again possible for a engine rebuild where the builder messed up. if the builder setup the rockers loose because he was suppose to make adjustments later and he forgot that is an issue.

a bad electrical problem will also look like a burnt valve or a sticking valve on the vac gauge.

your compression test sounds good, if the valve was not fully closing on some of the heads then the compression readings would be all over the place indicating a problem.

i'm leaning towards problems with the heads on the engine with the stem seals.

The best would be to try and get together with the engine shop. if the car is drivable then take it over to them and explain the problems and let them look it over. you have a new engine so its obviously not right and they should be at least willing to help you and diagnose it.

the other end is going it alone and swaping out parts and trying to narrow down things. but again i suspect 2 issues that are overlapping a little. ignition is showing a problem on the plugs and smoke from the valve cover vent is showing a seal problem.

also changing the coil would be a good test.. since after 5 minutes it loads up and dies that could indicate a bad coil as well.

 

[droptop73]

Smoke from the crankcase could be blow by where the rings have not sealed. It depends on what type of rings were installed and the bore finish to determine how long they take to break in. That still does not address the erratic vacuum readings and rough idle. It sounds like 2 different symptoms, idle quality and smoke from the breather. I would recommend working on the idle to begin with.

Is the idle steady or does it fluctuate? Is it just the idle that is rough or does it continue up the RPM range? If it smooths out what RPM? Does spraying carb cleaner down the carb help? This indicates a lean condition that means an air leak somewhere or a significant carb tuning problem.

 

[Steve73QMach1]

Smoke from the crankcase could be blow by where the rings have not sealed. It depends on what type of rings were installed and the bore finish to determine how long they take to break in. That still does not address the erratic vacuum readings and rough idle. It sounds like 2 different symptoms, idle quality and smoke from the breather. I would recommend working on the idle to begin with.

Is the idle steady or does it fluctuate? Is it just the idle that is rough or does it continue up the RPM range? If it smooths out what RPM? Does spraying carb cleaner down the carb help? This indicates a lean condition that means an air leak somewhere or a significant carb tuning problem.

Jeff - the idle is not steady. After the car reaches operating temp, it will idle on its own for about 5 minutes. During that time the vacuum is jumping all over the place (14 to 18" or so). If I hit the gas, it smooths out at about 2500 RPM and vacuum is good at 20". Having said that, the car has little power while driving in that RPM range even though it is smoother. I won't get to work on it until this weekend but I'm going to do like 72H says and get the plugs gapped correctly, reset timing, and see where I'm at then. Next step is to go back to the original coil. One step at a time... Thanks for your help!

 

[wwhite72]

You didnt mention ignition setup, assuming your coil matches the voltage of the ignition. That is to say a points setup and a pertronix I are same type coils. The Pertronix II and MSD ignitions use a different (higher constant voltage) coils. And they are NOT interchangable for more than a few hours of running time. Just saying, eliminate that possible problem.