Hi everyone.

Well, made a discovery on my engine that I'm not too pleased about.


First off, a little back story for any who may have missed it. A month or so ago, I went and checked out an '86 F-150 with 300 in it and bought it for parts for $300. Before I even thought about buying it, I ran a full compression check on the engine. All six spark plugs looked perfectly healthy. (I still have all six of them in my tool box and they look great). The compression test came out as ~130psi on all six. Lowest about 125, highest about 132. I fired the truck up and drove it about 20 miles (some interstate) to my brother's house. It drove just fine and I never noticed anything off about it.

So, a few weeks ago, I swapped the entire drive train over from that truck onto mine. Transfer case, transmission, engine. I pulled my engine out so that I could rebuild it since it was clacking and making all sorts of noise.

AS SOON AS I fired it up, I noticed something odd about it. The exhaust was popping and the vacuum needle was bouncing erratically between 14 and 16psi. I took it for a drive and it was low on power, sputtered, and the harder I pressed on the gas, (and the closer to 0psi the vacuum needle would go) the more wildly it bounced, like from 2 to 7psi. On the flipside, the higher the vacuum goes (like around 18 to 22 when I let off the gas) the more steady it is.

There's no backfiring, no stalling, it starts fine, drives fine, just a little weak, the exhaust sputters, and has a dancing vacuum gauge.

Well, today, I pulled the spark plugs out and 5 of them looked great, but cylinder #5 was black. Not oily. Just like it was the richest you could get it. I grounded it and checked for spark, and it was getting spark.

Did a compression test on the cylinder and it was around 30. The odd thing was, when I'd turn the engine over, the needle would bump up once (to around 30) and then wouldn't even twitch. I would normally thing it'd slowly bump itself up to 30, or would jump to 30 and then at least blip every time I'd fire it, but no.

What would cause this? As said, compression was great when I went to look at the truck. Drove it 20 miles, pulled it, put it in mine, and as soon as I fired it up, it had a dead cylinder.

Thoughts?

 

I can sure understand how you feel after just going thru an engine I had recently bought.

My first thought would be to pull the valve cover and see what seems to be happening with the valves as it rolls over that hole..then check the lash on each valve..make sure they're seating good. I have actually bumped them with a hammer to make sure they're not sticking...then do a leak down test...you'll find where the air is going...if it builds any pressure at all above 30#

I'm sure you'll figure it out..

 

I plan on taking the valve cover off tonight to get a look at what is going on. Just curious if, by the symptoms, someone would immediately know what it was.
The sudden loss is what gets me, since it was as soon as I fired it up in my Bronco. Yet, the compression test was just fine in the guy's yard.

My thoughts are either a really stuck valve, or a broken valve spring.

 

Sounds like a valve. Might be a chunk of carbon on it.

 

If there were a chunk of carbon on it, how would one fix this without removing the head?

 

If the valve cover is off, you can pull the rocker arm to the side, and hit the valve with a no bounce hammer.

I have seen people pour water slowly through the carb, holding the throttle open.

We used to just take them out, and run them hard "to blow the carbon out of them"

 

I'll try the hammer when I take the valve cover off. Ordered a set of springs for it "just in case" ($5 for the pair, why not), and no point in taking it off until they get here.

In the mean time, I sucked a bunch of Seafoam into it through a vacuum line, let it sit for five minutes, started it, filled up 5 driveways full of white smoke (mmmm, smelled like BBQ), and then took it out and ran it really hard. It ran a bit smoother, but didn't fix the issue at all. Vacuum needle still bounced erratically, exhaust still pops.

Oh well, I'll find out more when I pull the cover.

 

I can't imagine bending, or burning a valve in that short amount of time. It has to be carbon on a valve, or a spring.

 

The speed that it happened (20 miles?) makes me think a spring. 130 psi to 30 in that short of time isn't because something wore out. Plus, while I had the cover off putting on new gaskets, I turned all the push rods and none of them were bent.

Now that I listen, there's a very light tick coming from the #5 area (the one with the problem) of the valve cover. I'm sure I'll be able to see something once it's off. I'll report back.

 

My 89 spit a rocker arm off one time. I bet that's what happened.

 

Here's what it did, the rocker arm wore, and it shot out. I just replaced that rocker arm, and push rod

 

I think I remember you sending me pictures of that. Fortunately, from when I worked on my other head, I still have 2 of those pedestal channels sitting in my tool box should I need one (since they came in a pack for a V8).

 

Yep, those were the ones. Sure would be a simple fix.

 

I hope that's it! I let that truck set for a whole year, because I thought it ate a valve!

 

I've done this plenty of times and it works great. Steam cleans the engine, if you will. Transmission fluid also works great but is a pain if you accidentally stall the engine. With water, if you stall the engine you simply start it back up. With transmission fluid, a lot of the time you'll have to pull the plugs to clean them.

AB, make sure to let us know what you find out.