Hi all,

As some may know, I bought an '86 parts truck a month or two back and swapped the engine and transmission into my Bronco. It is BY FAR the smoothest and quietest engine I've ever had. It's a real beauty.

Oddly, though, it just has low engine vacuum and is just overall weaker than the other engines I've had in it.
I ran a compression check yesterday after it'd been fully warmed up and driven:

1 - 152
2 - 150
3 - 142
4 - 151
5 - 148
6 - 150

Not super tight, but not bad at all, with a 7.5% variation.

However, when I start it up in the morning and it's cold, it idles around 14 - 15hg on the vacuum gauge and when it's fully warmed up, it's around 15 - 16hg. Driving around town, it's generally 3 - 4hg lower than it used to be with the previous engine was at given speeds/conditions.

The previous engine rattled and made all sorts of noise, it had oil around the spark plug holes from bad rings, etc. etc. but it was a brute. The spark plugs generally had buildup on them and never really looked healthy.

This engine is the epitome of glass smooth and luxurious. But it just seems....weak. All six spark plugs are nice and clean with a spot of healthy tan.


I've checked the distributor to see if it was off a tooth or something and as far as I can tell it isn't. It's around 10° advance (same as last engine). The harmonic balancer isn't slipping. Same carb, same jetting, same tuning, etc. I've checked it over several times with spraying carb cleaner around it checking for vacuum leaks and it doesn't seem like it has any at all. I've also tuned the idle mixture and timed for highest vacuum.

15hg idle just says something's wrong. My last engine was around 19 or so right before I pulled it out.

Any ideas?

 

AbandonedBronco,

This is puzzling! But, I am sure it will be figured out.

Of course, your engine has a 4 barrel and vastly improved exhaust and mine is essentially stock except for Duraspark2, but at idle [650 RPMs], mine will pull 21.5" of vacuum. I have it set @ 19.0" to eliminate pinging.

On the FS forum, recently, I read where FTF mentioned that on one of his vehicles, he noticed a decrease in power and traced it to EFI exhaust manifolds which had shifted and were partially blocking the exhaust ports. He made mention that the EFI heads which have the extra bolts [studs] are there to help hold everything in place. Most of us have always thought that the 3 (?) extra bolts were to facilitate the installation of the three manifolds on the assembly line [only]. Is it possible that your exhaust might not be properly aligned? He ended up fabricating small pieces of channel which when bolted to the exhaust studs, actually held the manifold in place, rather than the normal round washers.

 

It is puzzling!

I remember reading that post and thought it was rather interesting. It could be a good idea to create a bracket of some kind.

Although, I don't believe I've ever had a manifold slip down and have put mine through some pretty rough roads and abuse, and they're always in the same spot. The exhaust pipe is really solidly in place and I can often just rest my manifolds on top of them when putting everything in place.

And while I can see it causing a lack of power, I don't believe it would cause a low vacuum condition. It would simply simulate a restrictive exhaust.


One idea (which I hope it isn't!) that was brought about by something FTF mentioned was the cam being off a tooth. I wish I would have thought to check to see if the timing marks lined up when I put a new timing cover gasket on.

 

Hmm, advancing the timing didn't help? My 86 pulls ~22" idling at 650 with the Offy C, Edelbrock 500, and EFI exhaust with base timing at 12* BTDC.

That engine is just a stop gap until you get the old one rebuilt - i.e. not something you want to fool with too much?

 

If it's something that I can fix, I'd definitely like to. It is intended to be a stop gap, but if I can figure out the issue and then it runs really strong, I may just keep it for a while and rebuild the engine for the other Bronco (which has a knock in it).

If not, then yeah, an intermediary engine.

 

Bronc,
Just for giggles, check your vac gauge.
Have you run the engine long enough to see what sort of mileage it's giving?

 

I have the vac gauge in the cab with me at all times, which is what first brought my attention to something being off. 15 - 16hg idle is pretty low, and overall it's lower than what I'm used to going around town.

Don't know on mileage yet. I just replaced my fuel pump, which had a seam leak on the fuel line connection, so I was "using" a bit more gas than I should have.

 

Well, I pulled the valve cover off last night and watched the rocker arms as I rotated the crank.

As far as I can tell, the crank and cam are lined up right.
Just as the #1 piston was reaching TDC, the exhaust valve was closing and then both were closed at TDC. As it started to go back down, the intake valve started to open.


Not only am I stumped, but I'm also just really curious on this one.
I can't find any intake leaks at all. My timing looks good. My A/F mixture looks right.

The engine seems nice and tight and purrs like a kitten. But I've never seen 15hg vacuum at idle before on either of my 300s. What would cause it?

Any other ideas?

 

only a couple of things can cause a low reading with a steady gage reading. IIRC thats a vacuum leak, timing issue or bad ring/s and since its steady the A/F is probly ok as well as the valves.

did you check the brake boster out?

 

I just can't seem to find any leaks. I doused the intake, carb, and base plate with carb cleaner and there isn't so much as a hiccup in the engine for a wiggle of the vacuum needle. I'm coming up empty handed each time.

I haven't checked the brake booster, it's brand new (about 2 months maybe), and if it was causing issues I would have seen it on my last engine too. I had about 19hg or so on the engine before I swapped it a few weeks ago.

Wouldn't bad rings show up on a compression test?

 

If it's an 86 motor, are all the rocker arms torqued to 17-23 ft lbs?

Is the idle set right? Are the secondaries open at idle? Does the choke come off fully?

Is the PCV clogged?

Undo the line to the brake booster and plug the port for it to see if your brake booster is leaking.

Could be anything.

Have you done a leak-down test yet?

 

I've checked the distributor to see if it was off a tooth or something and as far as I can tell it isn't. It's around 10° advance (same as last engine). The harmonic balancer isn't slipping. Same carb, same jetting, same tuning, etc. I've checked it over several times with spraying carb cleaner around it checking for vacuum leaks and it doesn't seem like it has any at all. I've also tuned the idle mixture and timed for highest vacuum.

AbandonedBronco, the above quote was from your original post. It looks like everything has just been swapped from the original to the replacement engine... was the distributor also swapped?

 

Oh, yes. The only thing that changed was the long block. Everything else was just unbolted, set aside, and then put back on.

 

1983F1503004X4 asked if the PVC valve was clogged... Did you by chance put a new one in or did you also swap that from your previous engine setup?

 

Thanks for the ideas.

Yes, all the rocker arms are torqued to 20 ft/lbs. I went over and retorqued them a few weeks ago.

The idle is adjusted and set for the highest vacuum reading. It idles super smooth at 750 RPMs. When it's warming up and idling at 1500 RPMs, it's around 17 - 18hg. On my last motor, it would be up in the 20s.

I haven't checked the PCV. It may be. But, in times past, I've capped off the PCV with my hand and never noticed a dramatic drop in vacuum. I can check it though. The valve is only a year or two old.

I can check the brake booster line but have no reason to think it's an issue. The booster is only a few months old and wasn't causing any problems with the old motor.

I've only done a compression test. I haven't done a leak down test yet, as I don't have the equipment (and it's a bit more expensive than a compression tester).