Re: [NFC] Chevy small block V-8 TBI - stalling issue
From: Lashdeep Singh (lashdeepyahoo.com)
Date: Wed, 12 Sep 2018 15:55:25 -0700 (PDT)
Peter, have you confirmed fuel pressure readings on an accurate auxiliary gauge?



On Sep 10, 2018, at 02:02, Peter Rychel <dino308gt4 [at] hotmail.com> wrote:

To change things up a bit, here’s a tech issue I’d like to have some feedback on. I remember that a few Listers have some American iron in their fleets of cars, so maybe someone can suggest something I would have missed while I’ve been diagnosing this problem on my own. Sit back and bear with me with this long description (I don’t want to leave any details out).

 

The vehicle in question is my 1990 Chevy Sierra pickup with the 305 TBI engine. It was our old shop truck that they were getting rid of and I bought it because I got it for literally nothing and figured it wouldn’t take much to revitalize it. Other than a leaky waterpump and a smoky exhaust, it ran well enough that I decided to pull the motor and rebuild it.

 

The bores were worn, but not damaged, so I had it bored out first oversize (actually just below as 20 thou cleaned up all eight and I got a piston and ring set in the respective size). Journals were all good so just freshened it all up with new bearings (big & small ends and cam). I used Sealed Power brand everything (cast alu. pistons, stock CR, iron rings, tri-metal bearings. Nothing fancy or expensive).

 

The top end I did a bit of work on. I cleaned up the exhaust ports just by smoothing out the rough casting. Did not open it up. Intakes were left alone. New valves, stock sizes, kept the exhaust valve rotators and only upgraded to Camaro Z28 springs (only slightly stiffer/harder than stock). Each valve was lapped during assembly only, I didn’t go fancy with multi-angle seats.

 

In respect to modifications, I installed a Comp Cams cam rated for street, TBI engines. The stock cam was worn out so I figured if I’m going to buy a cam anyways, I’d go with one that offered a bit more kick. Again, not anything radical, just a mild improvement over stock with moderate increase of duration and lobe center angle (LCA was as stock with this grind to keep the vacuum signal for the ECU happy). They claim it won’t disrupt the stock engine ECU. I also installed the recommended Comp Cams hydraulic, non-roller lifters and kept the push rods. A new timing chain and sprockets were installed too. Because the stock cast-iron exhaust manifolds had a crack in it, I picked up a set of headers off of ebay that were less expensive than the stock iron ones. The rest of the exhaust is stock in respect to pipe diameter (stock Y-pipe, deleted the cat and a Thrush Welded muffler). The headers came with an O2 sensor bung in the stock location and I got a new sensor to go in it’s place (along with every other sensor on the engine).

 

The distributor got a new pickup coil and amplifier unit. Shaft ran true with no wobble. I kept the cap and rotor as they were changed before I bought the truck. There’s new plugs installed and new wires.

 

The throttle body saw the most work. I installed a spacer plate below it and worked on the openings, by milling off the intake horns and smoothing out the profile to a nice, bell shape. I also installed a fuel injector pod spacer plate and re-worked the air box to provide unobstructed airflow (which was always a criticism of the TBI system). I kept the crankcase breather hose into the airbox, as well as the PCV routed into it’s respective port in the throttle body.

 

The EGR was retained (new unit) as well as the stock intake manifold. All vacuum lines connected with fresh, new, hose. Heat stove pipe for the intake is in place and opens up gradually as the engine warms (as it should).

 

Ran the engine in per camshaft recommendations (2000 – 2500 RPM for 30 minutes using Comp Cams break-in oil). Did the recommended oil change immediately after and using AC Delco E.O.S. additive with mineral 10W30 Quaker State currently. Absolutely no consumption and oil is as clear on the dip stick as it went in. No fuel smell in the oil either. Zero soot on the tail pipe outlet. Exhaust smells clean despite the lack of a cat.

 

So my problem? If I stab the throttle hard enough, the engine coughs and stalls dead. Pressing the throttle very slowly, it’ll rev fine, in fact all the way to redline (4800 RPM) and beyond. Idles smoothly, doesn’t hunt, buck or shake. No change in symptom when the engine is cold and first started up to when it reaches running temp.

 

My attempted fixes so far were to get rid of the stale, old gas in the tank which was the lowest grade (87 octane, 10% ethanol – this project has dragged on for months as I’ve been doing it off-and-on when I’ve had free moments which isn’t very many) to non-ethanol 91 octane and also the fuel filter. Just doing these two things have dramatically improved the running of the engine but there remains this stalling problem. While the truck is driving down the road, pressing the gas sharply gives me nothing, just flat and gutless and only increases speed gradually. If I press the pedal slowly though, it increases speed progressively.

 

So, what should I check next? The fresh gas and fuel filter fixes made big changes so I’m leaning towards this being a fuel problem, maybe weak delivery (pressure/volume?). I seem to remember we had the fuel pump changed in this truck years ago, but not 100% sure. But could there be something else? I used the truck’s fuel pump to pump the old gas out – I connected a hose to the line and hard-wired the pump relay to run it. 110 liters in about 30 minutes, give-or-take.

 

No CEL on the instrument panel either, which is curious as I’m sure it would’ve tripped with each stall.

 

Hmmm...

 

Peter

 

Sent from Mail for Windows 10

 

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