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Rough Idle & Stalls

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Sep 25, 2012, 5:02 PM

Post #1 of 3 (1951 views)
  post locked   Rough Idle & Stalls  


Owned since new: 92 Chevy P/U C1500 - 4.3 z motor – TBI fuel injection - 2wd - manual trans - 325,000 miles - Old & Ugly but normally runs great and very reliable; had recent tune up of plugs, wires, cap, rotor, pcv, air filter


Became harder to start 3-4 times, then very rough idle & stalls, had to pump pedal vigorously to keep from stalling, won’t accelerate in neutral and wants to stall when pedal depressed. No check engine light.


My mechanic of 10 years said fuel pump was bad. He cut the rusted sending unit from the rusted fuel lines, said it was not worth fixing, and told me to tow it away with the fuel tank in the bed.

Replaced the fuel pump, sending unit, and fuel lines myself in my driveway.

Had same starting conditions as above. I used Chilton manual and multimeter to test TPS w/ key on (had 5 v on feed wire, low 0.45 v on signal wire at idle should be 0.5 – 1.2 v, signal wire went smoothly from 0.4 - 4.5 v and back down with key on); IAC valve had normal Ohmic readings; MAP sensor had normal 5.0 v on feed wire, totally abnormal voltage readings of 4.6 with key on and while running had 4.6 - 4.9v from idle through high acceleration (with hose disconnected). MAP hose is intact and has good suction.

When I disconnected MAP sensor from hose and electrical connector it started right up w/ check engine light on, accelerated powerfully in neutral, idles normally for a minute or so but will stall without a quick pedal pump, then back to normal for a minute or so . . . .

Replaced TPS and MAP sensors.


Still have same problems I started with and no check engine light. This time a small flame shot up from the TBI. Please help.

(This post was edited by Mikeymik on Sep 25, 2012, 5:05 PM)

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Sep 25, 2012, 5:12 PM

Post #2 of 3 (1942 views)
  post locked   Re: Rough Idle & Stalls  

That is a whole lot of miles if that is the original untouched engine. Might want to run a compression test on it to see what kind of compression it can produce. If the engine is old and tired it is going to affect the operation of the fuel injection system.

What is the fuel pressure? What is the fuel pressure when it is stalling or won't start? Is their a cone of fuel spraying out of both injectors during cranking when it doesn't want to start? You have to install a fuel pressure gauge in line after the fuel filter. Did you make sure you didn't get the supply and return fuel lines switched during your repair?

You shouldn't pump the gas pedal during cranking on a fuel injected engine. Your just confusing the computer by doing that.

Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.

(This post was edited by Discretesignals on Sep 25, 2012, 5:14 PM)

Hammer Time
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Hammer Time profile image

Sep 25, 2012, 6:49 PM

Post #3 of 3 (1921 views)
  post locked   Re: Rough Idle & Stalls  

This is a duplicate question. The original was moved to the engine engine troubleshooting section. Please don't create duplicates.


We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.

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