1994 K1500 p/u 5.7L missfire/hesitation

1994 K1500 p/u. 5.7 motor, auto trans. Problem with smooth starting and smooth takeoff. This problem just gradually surfaced over the past few weeks. prior to this, the engine ran just fine.
Truck needs feathering of the gas to keep running on a cold start (warm start okay). When taking off, engine hesitates and stumbles until foot is almost to the floor, then it takes off fast. Once it hits 35mph, it runs well with passing power, but not quite the power potential that it should have. This problem happens hot, warm & cold.
Timing set a 0 degrees with timing wire under glove compartment disconnected. Recent replaced parts include Fuel filter, ign coil, cap, rotor, wires, plugs, ign module. (no change) Fuel pump pressure checked at 13.5lbs good. Injectors are spraying good & evenly. TPS checked out okay. EGR works fine.
Plugs look great and even across all cylinders, and are not cracked. Exhaust is clean, no smoke.
Idle is normal with some slight erratic and non-rhythmic stumbles while in gear & stopped. When in park, and gas is stepped on, engine stumbles until it hits 2300RPM, then cleans out. No vacuum leaks.
Since this happens cold & hot (worse while cold), I'm taking in consideration that the problem exists outside of closed loop, which should rule out O2 sensor. This problem is intermittent, but it acts up more often than not. Since it "wakes up" at 35mph and above, I'm ruling out a fuel problem. As I mentioned earlier, timing is set fine.
So, I'm thinking ignition problem. I have not looked at the pickup coil, but that's one of the last things I have not touched yet.
Opinions???
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You might want to check/replace MAP sensor as it helps set baseline mixture. Have you checked/changed the gasket between TBI unit and intake manifold? Also, on a side note, those TBI 350's wake up a lot if you set timing to 6 or 8 BTDC and run at least 89 octane too. ----------------- TheSnoMan.com
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Hi:
I'll take a stab at this, but. It's like asking a doctor to cure a headache in the stomach, over the phone.
One of the very few things that is consistent in either closed or open loop, is the distributor pickup and the shaft.
He HEI design 3 that you have, is known for bad pickups and distributor shaft magnets.
Other than giving me scanner data, i.e..; long and short term fuel trims, and O2 sensor readings. This is a stab in the dark.
The nipple that the map sensor connects to, may be blocked with carbon.
I hope this helps.
RK
PS Leave the timing at 0, because it undermines the whole reason for computer controls, and is hard on catalytic converters.

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This sounds similar (but not identical) to an issue I'm having with my '93 K2500 with a 5.7. I've ruled out the MAP, catylitic converter, fuel filter, computer or and pickup in the distributer since I'm on my 3rd one. No codes in the computer. I know my fuel pump has issues (4lbs and sometimes stops) and suspect the injectors might be not so good after 230k miles. I am replacing the FP this week.
Good luck - sorry I don't have anything to offer, but I'll be watching this thread to get further ideas on what to do with my own truck.
--Jeff
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I'd check for a bad coil pickup. Hook a timing light and look for false triggers. Easy enough to check. Had this problem, and replaced the pickup coil to fix it.
Cheers
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What do you mean by look for false triggers? Watch the light while the truck is at idle and look for "extra" flashes? Or do it with the coil wire disconnected while cranking and watch for the same? As I said, I have similar problems (and am on my 3rd pickup) so I wouldn't mind testing this out again just to be sure this latest pickup is solid.
Thanks, --Jeff
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wrote:

<snip>
Exactly, The light should only trigger on TDC. And dont forget to pull the brown timing wire in the cab to disable the advance.
Cheers
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If you're on your third pickup:
Then the magnet on the shaft must be bad.
Look at the star shaped points on the shaft, and if you see like a greasy substance on the edge of a point.
The rubber magnet has melted, and you'll need a new shaft.
RK
wrote:

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Just added to the mix.. Replaced MAP sensor. No difference. No leaks around the TBI. No intake leaks (corners or ends either)
Timing at 6-8??? Sorry to say Snoman, but you may be on drugs. If this was a typical Gen I HEI, then okay. But not on these kind of distributor/timing systems. These systems can only take 2-3 degrees at the max. Baseline is 0 with the computer timing plug unplugged.
I'm about the change out the distributor. If that does not work, then I'm going Gen I HEI and carb.
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Distributor changed. no significant change..... except that I can now feel a slight cylinder drop when I first hit the gas.
My guess is to take compression checks, check rocker stud height, check for broken valve springs.
Stay tuned.
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If you pull the plug wires one at a time:
You'll see the effected cylinder.
If you have access to a leak down tester, do a leak down.
Also, consider doing a running compression test, that will show sticking valves and weak springs.
RK
PS You do a running compression test by: removing the Schrader valve from the compression tester, where it threads into the cylinder, then test a cylinder on the other side for a comparison.
wrote:

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If you do that:
You'll fail even a visual I/M inspection.
RK
wrote:

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UPDATE..... 1994 K1500 p/u. 5.7 motor, auto trans. Problem with smooth starting and smooth takeoff. This problem just gradually surfaced over the past few weeks. prior to this, the engine ran just fine.
Truck needs feathering of the gas to keep running on a cold start (warm start okay). When taking off, engine hesitates and stumbles until foot is almost to the floor, then it takes off fast. Once it hits 35mph, it runs well with passing power, but not quite the power potential that it should have. This problem happens hot, warm & cold.
Timing set a 0 degrees with timing wire under glove compartment disconnected.
Recent replaced parts include..... Fuel filter, ign coil, cap, rotor, wires, plugs, ign module, MAP sensor, knock sensor, entire distributor, pickup coil, magnet. (no change)
Fuel pump pressure checked at 13.5lbs good. Injectors are spraying good & evenly.
TPS checked out okay. EGR works fine. Sprayed around TBI for air leaks... none. Intake manifold is tight.
Took off valve covers, checked for broken springs... none. Went through valves for tight loose ones... none.
Compression test was good and even between all cylinders.
Plugs look great and even across all cylinders, and are not cracked. Exhaust is clean, no smoke.
Idle is normal with some slight erratic and non-rhythmic stumbles while in gear & stopped. When in park, and gas is stepped on, engine stumbles until it hits 2300RPM, then cleans out. No vacuum leaks.
Since this happens cold & hot (worse while cold), I'm taking in consideration that the problem exists outside of closed loop, which should rule out O2 sensor. This problem is intermittent, but it acts up more often than not. Since it "wakes up" at 35mph and above, I'm ruling out a fuel problem. As I mentioned earlier, timing is set fine.
My Actron is reading the ECM fine (for what an ALDL is worth). I swapped ECM's with a friend's truck (same year, etc)... no luck.
Now, I'm really to the point of pulling the ignition system, and shoving in a HEI setup. I'm trying to investigate if the fuel system can work on its own with the wrong ignition system in the truck.
I don't care about the engine light.. as long as it works. (by the way, we don't have inspections out here in the country. we have the freedom to do whatever we want to our vehicles)
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BLB wrote:

SNIP
Sounds like a fuel problem. Pull the vacuum line to the brake booster and take a sniff. If you smell a strong gas smell check the fuel pressure regulator. It could be leaking through the diaphragm causing a rich condition when it has a higher vacuum condition. This would clear itself when the engine starts burning off the extra fuel.
--
Steve W.
Near Cooperstown, New York
  Click to see the full signature.
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Fixed.
It was the fuel pump. At idle, pressure was fine, but it could not hold up on load. I failed to monitor the pressure under load when I tested the fuel pump pressure. A friend told me to spray carb clean down the TBI, as I jammed on the throttle. If the hesitation goes away, then the problem is the fuel pump. Sure enough, it was. He also recommended a GM fuel pump ONLY. $80, lifetime warranty.... and it's working great.

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