Question about 1999 GMC Sonoma

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My 1999 GMC Sonoma(manual transmission) sputters

My two wheel drive 1999 GMC Sonoma(4 cyl, 2.2 liter, manual transmission) sputters when I let off the gas to maintain a speed. If I let off the gas all together or accelerate it runs smooth again. I let the engine warm for 5-10 minutes in the morning but still the problem persists. Also, I find it's easier to start when cold than warm outside. Any ideas?

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Yes, I have a 94 GMC Sonoma that exhibited these exact symptoms. Even after replacing the pump twice, the truck would run ok for maybe 3 to 4 hours and hitch up and die. The last time it happened, I was SURE that it wasn't the pump, (lol) as it was brand new, from a familiar uto parts store. Here is a list of things I attempted to do to bring it back to life..I live in a very cold climate and having a running vehicle is serious business here, breaking down can go from an annoyance to life and death within a few minutes. This was a NIGHTMARE.. Replaced; Fuel pump; rusty fuel lines and filter; coils; control module on the engine block; PCM; battery; re-grounded everything repeatedly. No change.


1. Checked voltage at the fuel pump with a multi-tester. Its a grey wire that runs under the drivers door inside the frame. My truck is old and not very pretty. I took an air chisel and made a hole over the sending unit in the bed, because I was very tired of removing the bed every 2 weeks to mess with the pump. I made a hatch cover that bolts into the bed with a couple of tacked nuts (migs are great...DO this with the bed OFF, keeping in mind that this is the LAST time you'll being doing this)). I DID have 12 volts at the pump, Truck would start and run for roughly 3-5 minutes and die like someone had cut the key switch off. I pulled the sending unit out of the tank. There is a small plastic insert that the wires run through, into the sending unit. Underneath, there is a small retaining clip that holds this plastic grommet in place. When I removed the clip (carefully! we must reuse!) and pulled it apart, there was a large buildup of rust between the pins on the plug. What a MESS! Not a direct short, but wet rust...There is an o ring that fails and water gets into the grommet. Weak point one. Several Auto parts store now offer the pigtail and grommet separate from the pump kit, so while gm may say its not common, demand dictates sales....anyway....
2. Aftermarket fuel pumps from your local hardware/auto parts place usually come with a separate strainer for the pump. SPEND THE 8 BUCKS. Its internal, meaning it is the fuel pump's first line of defense. Manufacturing isn't always exact...The foot strainer had fallen from the pump and into the tank to float around. Big problem..pump drinking tank rust is bad news. Bend the tabs on the foot strainer in a hair and make sure its gripped onto the pump. Also check the rubber line in the sending unit if there is one (not all have it, ive seen both in this year truck.) They can crack or burst when they get old, and you lose fuel pressure, even though its spraying back into the tank, so its not readily apparent.
3. Bad or no oil pressure WILL cut fuel delivery to a chevy 2.2 engine. If this is the case it will not start at all.


4. When it cuts out and dies, pull off the road somewhere and reach under the truck, passenger's side. Feel the heat from the exhaust in front of the catalytic converter. Just hold your hand about 2 inches from the pipe, not touching. Now feel behind it. Is there a huge difference? If so, clogged cat can make these run like ****.
5. Timing. I was getting frustrated by this point. I tore the front of the engine down and looked. timing was fine, although no timing mark on the crank damper means no timing light. BUT. I have heard from very experienced mechanics that these CAN jump one tooth, and the computer compensates the timing as much as possible. Which means..while It will still run, it will have no power, to the point where giving it any gas at all results in stall condition.


6. THIS IS WHAT FIXED IT. I borrowed some tools from a friend which gave me the final verdict..by this time a month has passed, I'm $500.00 poorer and still no vehicle. I borrowed a fuel pressure tester and a compression tester. 2.2's are known for weak head gaskets, they get something called siamese cylinders. this means they are weak between number 2 and 3 pistons, and they start sharing compression. Checked that, all was fine.
DID fuel pressure test...(these engines require fuel at a minimum of 41 psi for the injection to work properly.) My fuel pressure test resulted in a disappointing 18 psi!!!! and whats worse...after shutting off the engine, the fuel rail quickly lost pressure. Within a minute dropping to a measly 5 pounds. I was getting that sinking feeling again..another fuel pump, and how long will it last. :( I'm fairly sure it got debris in it and chewed itself up, but cant be certain.
I went and bought a new fuel pump and strainer, and changed it again, 3rd time. This time the truck starts and runs for 10 minutes or so and doesn't die. I shut it off, and try to start it again...and it cranks hard for 10 seconds or so before reluctantly starting. And yes I had even let the pump prime first. So, this tells me that the fuel can only go one of 3 ways from the fuel rail where I'm testing it.
A. Into the cylinders. And if its losing pressure in the fuel rail that fast and dumping into one cylinder (a damaged injector seal), it would puff black smoke when it started again, and smell like raw fuel. Or possibly dump enough fuel to hydro-lock the engine. And ruin the oil. This wasn't happening.
B. Returning to the tank through the pump. Highly unlikely. These sending units have one way valves built into them to prevent the pump from losing its prime and doing dry starts. Rare.
C. The fuel return line. This line is fed by the FUEL PRESSURE REGULATOR. The fuel pressure regulator opens and closes the return line through manifold vacuum. Check to make sure the F.P.R. is not leaking gas into the vacuum line. Mine was not leaking fuel. Replace the vacuum line to the regulator.
CLAMP THE RETURN LINE CLOSED WITH VICE GRIPS on a RUBBER section of the line (most likely back towards the tank itself), and re-do the fuel pressure test. See if it holds pressure in the fuel rail after its shut off.


IF IT HOLDS PRESSURE, THE FPR IS BAD.


IF IT DOES NOT HOLD PRESSURE AND THERE ARE NO LEAKS, YOU HAVE A BAD SENDING UNIT or a BAD INJECTOR SEAL.


Changed f.p.r. on my 94 Sonoma..this truck had problems for years with hard starting after it warmed up, eating pumps like candy, sluggish/no acceleration (even for a 2.2 lol).........EUREKA!!!!!!!! No more buying pumps every week! Truck starts hot or cold now, and it doesn't load up when you let off the gas like it did. I could actually tell by the sound that the new fuel pump was happier than its unfortunate ancestors. I hope this helps you or anyone who reads this, so that you don't spend 2 months puzzling over it and spend around 500 on pumps. The new pump was 94$ with the strainer, and the fuel pressure regulator was I think around 50$. I see countless endless complaints on forums about S-10s and Sonoma's 2.2 engine eating fuel pumps, so I just wanted to share my experience with you guys, its been a journey. In hindsight, thinking this through. Usually a bad fuel pump is caused by a restriction in the fuel system which fights against the pump (i.e fuel filter clogged, pinched line, ect.). The reason this was so hard for me to pinpoint was because of my old school mechanic mindset. The ACTUAL cause of the fuel pumps being overworked was, that they had to feed 4 injectors at 41psi, while ALSO compensating for a malfunctioning regulator that was remaining OPEN. Kind of like blowing up a balloon with a pin-hole in it. NOT A RESTRICTION.........A LACK OF ONE. :) GOOD LUCK :)

Posted on Jan 19, 2013

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Change fuel filter, plugs and wires.
Check for any vacuum hose leaks.
Check gas cap to make sure seal is not broken
Check fuel pressure and spark to all cylinders.

If check/service engine light is on, have an engine diagnostic test done to pinpoint exact problem.

Posted on Nov 16, 2009

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