TBI 454 lacks power

[Dr_Snooz]

Thanks. Part of the diagnostic was to check for RTV contamination of the O2 sensor, which shows up as a white powdery buildup. Mine looks okay to me.

And yeah. I was checking exhaust back pressure. It was hard because the needle on the vacuum gauge was gyrating over such a wide range at higher RPMs. The vacuum never fell off like with an exhaust restriction, so I'm not worried about that. I'd take a video of the needle movement, but I misplaced my gauge. I'll do that when I find it again.

I talked to my rebuilder the other day about the possibility of weak valve springs. He checks valves during building with some kind of special tool that compresses them and measures tension, so that kinda answers that question for me. He wants to put it on their scope to see what's up.

Will post again when I learn more.

[pholliday1]

Not sure if this is your problem, but in the past I've had a similar problem and the cause end up being the distributor. The bushing's we worn just enough to give these "gremlin" type problems and because the distributor still "works" you get these on again off again problems with no codes. And for what its worth also had an issue with a "known new good distributor" that was bad also.

[Dr_Snooz]

Update on my useless slug of scrap iron. I promised to take a vid of the bizarre vacuum signal I'm getting and here it is. I'm trying to figure out what it means and am open to suggestions. Timing chain off a tooth? Valves screwed up? Bad Feng Shui? What?

http://vid441.photobucket.com/albums/qq132/dr_snooz/MVI_1425_zpsxdqwnkhx.mp4

Bear in mind there aren't 10,000 miles on this engine. My new '89 Suburban TBI is rock steady at 19 w/ 90k or so miles. This engine flops all around and has no power at all.

[bd]

Unable to open the file.

[Dr_Snooz]

Well. Try these I guess.

http://s441.photobucket.com/user/dr_snooz/media/MVI_1425_zpsxdqwnkhx.mp4.html

[Dr_Snooz]

BTW, the MAP sensor tests good. The only thing I haven't tested is compression, which is next. Followed by something drastic.

[Dr_Snooz]

Here is my album. It's the first video.

http://s441.photobucket.com/user/dr_snooz/library/?sort=3&page=1

[bd]

Scroll down to the image of the gauge and 'click' on the various scenarios:  How to Use and Interpret a Vacuum Gauge.  Now, go back to 5.  Does it run smooth, or is there a little miss?

Where did you have the gauge plumbed in at the time?

[Dr_Snooz]

Now, go back to 5.  Does it run smooth, or is there a little miss?

I'm not able to look at the scenarios on that page. I had so much trouble with Adobe Flash, I had to uninstall it. Is Scenario 5 the "weak valve springs" diagnosis?

That's a wonderful site, BTW.

The idle is not right. It does miss, most of the time. At other times, it surges. The gauge was piped into the throttle body in place of the PCV hose.

A couple other notes. The truck runs fine at a steady speed on flat ground. When you get harder on the throttle is when everything unravels.

Sometimes, the cab fills with the smell of burning brown sugar. ?! Especially if I've been driving the truck under load.

If I mess with the vacuum, introduce a vacuum leak, bypass the PCV, etc., the truck runs slightly better for the next couple days.

I swear I'm not making any of this up.

[bd]

You should author a mystery novel....  Really....   

Scenario 5 is "sticking valves."  I wasn't able to get a steady stream via your link, but the ol' sticky valves routine seems the best fit - or similar valve issues.  I wanted you to see it and compare for yourself.  Does it pop back through the intake if you aggressively snap the throttle wide open?  The surging is probably fuel management trying to compensate for the misfire.  Are you sure the 'brown sugar' smell isn't coolant or oil coking on the exhaust?

[Dr_Snooz]

Okay, so it does seem like there is something up with the valves. What would make the valves sticky on a newly rebuilt engine? Is there anything I can check that doesn't involve a tear down? My rebuilder is super highly respected in town, especially among the farmers. He inherited the business from his dad and has been in business forever. He's very careful and I can't imagine anything meaningful escaping his notice. A bad cam lobe or bent valve or whatever would have been seen upon inspection and I would have received a call. The strange thing is that the truck passed CA smog with really good numbers running this way. Valve trouble would tend to prevent that, but this is so bizarre, who can say?

The truck has never backfired, even at WOT. I haven't tried blipping the throttle though, so I'll see. My hunch is that it will merely stumble and hesitate. I get a lot of stumbling and hesitation off the line. The truck has never lurched or bucked or backfired. It just has a subtle misfire and no power.

I think the brown sugar is a wire loom or relay cooking itself. I've spilled enough oil down the side of the block to know what that smells like and coolant smells different than this.

I'll do a compression test next. It's really hard to believe that this is mechanical, given that my symptoms aren't consistent, but I'm really out of ideas. Everything I test is dead perfect. The only thing wrong is this screwy vacuum signal and I don't know what it means.

[bd]

Burning electrical components produce a distinctive rank, almost rancid, odor.

Go ahead and run the compression and/or cylinder leak down tests.  They will tell you basic mechanical condition.  What will remain is fuel management and ignition.

What recent discussion had you with the rebuilder?  Did he ever check engine function using his scope?

[Dr_Snooz]

I went in a couple weeks ago to tell him what I'm dealing with. He went over some basic troubleshooting stuff, fuel pressure, vacuum leaks and the like, then promised to ask around. He called back a few days later having talked to a tech at the local dealership. He suggested some additional fuel pressure tests, which I haven't done. They are largely redundant, given that my pressure held at 11.5 psi, going uphill at hard throttle with the relief button on the gauge held open. His tech has to scare up his scope and they'll look at it then.

I told him about the vacuum shake and suggested weak valve springs. He did check the springs during the build and we didn't have a vacuum gauge to confirm the shake, so we left it there. He also wanted some compression numbers, so I have to get those now.

[Dr_Snooz]

Compression test results:

#1 - 110 psi
#2 - 122
#3 - 118
#4 - 119
#5 - 118
#6 - 121
#7 - 120
#8 - 111

That's definitely lackluster, but not enough to make my truck such a dog.

[Dr_Snooz]

Those compression numbers are underwhelming, but they aren't unacceptable. That alone shouldn't make the truck a complete dog. Something is going on with the valves, but something else is going on too in the engine electronics. My hypothesis is that the squirrelly vacuum signal is messing with the MAP sensor. I'll check valve lash next and hope it calms that vacuum signal.