Need Help with My '93 Chevy K3500 - 454 TBI problems

The 229,000 mile 454 in my '93 Chevy has been running on 7 or so cylinders for a while now.  The known issue is that the exhaust manifold gasket leaks horrendously, which melts the front left plug wire.  I've been procrastinating fixing this, as I don't drive the truck much, but I acknowledge it really needs to be done.

Saturday morning I get up with the sun to make the monthly trip to the local landfill.  Garbage is all loaded from the night before, I fire the truck up, it starts and idles fine.  Let it warm up for a minute while I get the recycling loaded, and head off.  As I attempt to accelerate (the truck is a 5 speed manual transmission) it bucks and snorts in every gear.  It seems down on power and doesn't want to go.  Complicating this, as the truck bucks it's hard to keep a steady foot on the gas, which induces still more bucking, so I have to clutch, let the truck coast and settle down, then give it some gas (and it revs OK not in gear) slip the clutch, and try to gain some more speed.

This is really terrible and I contemplate turning around, but decide I can nurse it to the dump and back, and plug onward, at 45 mph in the light Saturday AM traffic.  

15 minutes later, I get to the dump, and the truck is now, oddly, running better.  Which is to say, like a 229,000 mile 454 with a burned plug wire.  Dumped off the recycling and trash (didn't shut it off at all) and it drove home without bucking.  

Part of me says, ditch the stupid TBI, carb it, put in an HEI dizzy and be done with it.

The other part of me says, it's a TBI, it ought to be simple to fix, and if I ditch it, I may lose some other stuff that still works ion the truck. The speedometer is electronically controlled, as is the cruise (which works) and I don't know about the A/C (which also still works).

Recently replaced the Idle valve solenoid on the side of the TBI, the fuel filter, and did an oil change.  

Somewhat related, does someone reputable sell overhauled TBI throttle bodies?  Being a manual trans truck I'm concerned this one may be due for an exchange.  Also, when replacing the ide solenoid thing the tiny screws stripped out in the aluminum body, necessitating some "field expedient repairs".  

Here's a TBI exchange.

You won't know for sure if you need to do anything else to to the fuel system until you get the ignition working properly.

You have a problem that you already know about, don't go chasing anything else until you solve the first problem.

The TBI on my 400,000 mile 350 has never been touched, to my knowledge.

I was going to suggest a clogged cat...  But that will not clear itself out.  I thought that these things were somewhat bullet proof running wise, so I would not ditch the TBi.  You know you have an exhaust leak. Hose down the exhaust manifold bolts/studs with liquid wrench and have at it.  lol  I hope that fixes it for you.

Truth be told, there's lots of problems.  or, at least, potential problems.

The distributor cap is held on with zip ties (broken mount on distributor body)

The afore-mentioned cludge on the IAC due to stripped out throttle body

The afore-mentioned exhaust leak

I've just been nursing it along, due to laziness &/or other projects demanding attention, and like a good truck, it always starts and always runs and I keep beating on it.  I guess maybe it's time to load up the parts cannon and start dealing with some issues.  Starting with the manifold gaskets.  

And I really ought to replace that distributor, too.  I'm guessing OEM AC Delco will be the best bet?

In reply to noddaz :

No cat...P/O ditched it and it has a new exhaust from the crossover back, with a single side dump (3")

A bad enough exhaust leak will screw with the o2 sensor reading(s) and could be part of, or the whole problem. 

If you plan on keeping the truck long term, OEM distributor.  Possibly junk yard for the parts,  ie: throttle body.  Exhaust does play a role in the way "modern" vehicles run.

Check the distributor shaft for free play. The 92 454 TBI engine I messed with the upper bushing was quite worn and allowed the pickup teeth to touch. It had a misfire issue when the owner pulled the engine.

If everything else checks out, check the lifters and cam for wear. They were still using flat tappets until 1995, and cam and lifter wear can be an issue.

Did the exhaust manifold gaskets last night.  They were less of a PITA than I'd expected them to be; likely because the truck spent most of it's life in a salt-free climate.  Replaced the plug wires with some nice Belkin wires from NAPA, and replaced the TBI unit with a remanufactured one.  The truck fired right up afterwards, but soon went right back into the surging idle condition.  Mind you, it was much quieter now, but it still seemed to be "hunting" at idle- it would nearly stall, then rev up to 1200 RPM or so, then nearly stall, etc.  When giving it some gas, at first it revv'd up to around 4000 RPM, but then when I went to take it for a test drive, it would only rev to about 2500, in nuetral, just sitting at the end of my driveway.  

So I didn't even test drive it.  

I don't feel too badly about the TBI replacement- the old one had some other things going on I wasn't too happy with (dissimilar metal corrosion on the aluminum body to some metal fasteners).  

Last night, I loaded my wallet into the parts cannon and bought a new distributor and coil.  Again, I won't feel too badly about replacing these parts anyway; the current distributor has a broken cap hold down, so the cap wiggles a little bit, and it really needed to be replaced anyway.  And the coil was just cheap, so why not?

Vacuum leaks will do this every time on a tbi. 

Is the check engine light on? If so, jump with a paperclip and see what code. 

Could also be a bat water temp sender, but usually they just go to stupid rich.

No CEL.  Replaced every vacuum hose, hose clamped them, etc.  PVC is new.  Booster is hydraulic, so no vacuum there.  

There's some sort of vacuum-electro thingie with a small foam filter in it mounted up near the front of the intake manifold.  No idea what it does...but the foam filter was a bit damaged when i took it apart to clean it.  

Water temp reads OK.  Engine isn't running rich.  

Not being able to rev up to redline even in neutral is almost always low fuel pressure. 

Vigo said:

Not being able to rev up to redline even in neutral is almost always low fuel pressure. 

I have not yet checked fuel pressure.  I have read that that can be an issue on these trucks, too.  Fuel filter was recently replaced.  Seems like I'd better put a gauge on the pump and see what it's doing.  These are supposed to be about 14 psi, IIRC.  

Put the new distributor and coil in...didn't make a bit of difference. Still hunts at idle (down to almost stalling, then up to 1200 rpm or so, and repeating) and stumbles as it revs. I test drove it and was able to accelerate up to about 3rd gear. Then it had no more power. 

Now- did some troubleshooting. Checked the MAP sensor, it had ground and 5 volts, and the output varied from 5 to 1 volt as I applied vacuum with a mittyvac. So the map checks good. 

Checked the temp sensor- it had good ohm/resistance vs temperature. So it seems ok. 

Now- big clue perhaps: I disconnected the wires from the MAP and the engine idled properly. Steady, 800 or so rpm. But it throws a Service Engine Soon light. Plug in the map, light goes off, engine starts hunting at idle again. Took it for a drive with the MAP disconnected, and it drove much better, actually driveable, though the engine wouldn't Rev well- had to baby the throttle some. 

So, what's next? Going to see if I can check fuel pressure. 

Yep, fuel pressure and map sensor. TBI systems are very simple and reliable. It's easy to over think them.

1 other thing I forgot to mention- I disconnected the egr vacuum solenoid thing and applied the mittyvac to the egr valve while the engine was running. With a bit (2-3 lbs) of vacuum, the engine ran at a constant rpm, no more hunting, though it was a bit rough. 

I still think it's fuel pressure. The idle hunting could be caused by a lean condition. Disconnecting the map could possibly cause it to run richer at idle, making it run 'better' at idle but not addressing the  low ceiling on fuel at high rpm/load which is hardware related and not something that the computer can change. Using some EGR flow would also make it run richer because it would replace some of the burnable oxygen coming through the throttle body with inert gas from the exhaust. Given the same amount of fuel that would make the engine run richer as well. 

I agree with checking the fuel pressure - could a line have been crimped?

In reply to Vigo :

Agreed with your conclusions. I'm going to loan the fuel pressure gauge out from the auto parts store tomorrow. I checked the lines visually and they look good...suspect perhaps a weak pump. Filter is new. 

Feeling somewhat certain the issue is likely fuel pump related, last night I went out and peered under the truck to assess the situation with accessing the pump.  The truck has a 5th wheel attachment, which I'd hoped I could unbolt and have access to the top of the tank, but no dice- it was several feet away.  The pump itself was (of course) on top of the tank, and the bottom of the bed was approximately 2 inches above that.  So, to get to it, I'd either have to drain the 40 gallon tank, disconnect everything downstream, support, the tank, unbolt it, and drop it down...

Or, I could cut an access panel in the bed.  

5 minutes later, looking down at the fuel pump from the bed, I started cleaning off 25 years of grime and gunk from the top of the tank.   There were 4 wires leading from the fuel pump gas gauge sender unit dealy- 3 went to a plug, and one went to chassis ground.  Through a blue bullet connector.  That, when I tugged at it gently, the wire pulled off in my hand.  

Eureka.

I pulled the pump/ sending unit assembly out anyway, just to see what it looked like.  Surprisingly clean, though the strainer sock was somewhat brittle.  I ohmed out that ground wired and yes, it was the ground for the fuel pump.  Since I had the whole mess out anyway, and the pump is likely 25 years old and 230,000 miles old, and had been running with a compromised ground, and a new Delphi pump is like $60, and the sock is toast, I'm going to replace the pump while it's out.

Stay tuned...  

In reply to volvoclearinghouse :

Cutting a hole in this generation Chevy truck to access the fuel pump is par the course from what I understand - hope this fixes it for you! I feared the day my pump when on my Suburban, sold it before I had to deal with it. 

FUEL PUMP DID IT!!!

Got a new Delphi pump and strainer, stuck them in tonight, and just finished a test drive. Ran it up to 4k RPM in 3rd and it pulled strong. Idle is dead even. It feels like a new truck. Runs better than it ever has since I've owned it. 

Bonus: no angry texts from the wife to shut down the noise. The exhaust is subdued . 

Now I just need to figure out how to patch the roughly 7" x 12" hole I cut in the bed...

Congrats! 

In reply to volvoclearinghouse :

That sounds like the beginnings of an in bed beer cooler to me.