Chevy S-10 6cyl Pickup Hard starting when warm

My 1999 S-10 has some hesitation issues and my mechanic said I needed a new fuel pump. He put one on and since then the truck starts instantly when it’s cold but when it is warm and has set for a while it will crank for a while before it starts. Since then we have replaced fuel spider and regulator, fuel pump, cylinoid (sp) wire, heat sending unit, fuel filter and I still have the problem. I tried turning the key on before and hesitating before starting but that did not help. Mechanic checked pressure on fuel pump and that is not the problem. This seems to be a common with S-10 6cyl and there have been a lot of suggestions but as of yet no one has said that they have fixed it. Any suggestions or if you had this problem and fixed it I would be interested in what you did.
Thx in advance.

Have you tried holding the accelerator down 1/2" while cranking? Give it a try.

Did the mechanic check fuel pressure on it under conditions that produce this issue? Most fuel pumps have a check valve in them that keeps pressure in the fuel system after shut-down. If the check valve is a problem the loss of pressure can lead to the residual fuel inside of the rail vaporizing when things are hot under there. This will make for rough restarts. The way to find out is to put a fuel pressure gauge on it, get the truck up to temp and then shut it down and see how long it holds fuel pressure. It should hold it very well for a long time. If it falls off quickly then it is the fuel pump.

For priming the fuel pump, btw, turning the key to run and pausing a few seconds won’t do it. You need to go off-run-off-run-off-run a bunch of times. The pump only runs a few seconds for each turn, so if the pressure is low/zero it takes more than one priming run to get it all the way up to spec.

Cigroller, if the fuel pump was losing pressure why does it start instantaneously when the engine is cold?

As you stated, this is a common S10 problem. It seems replacing leaky injectors fixes the problem. I can not guarantee it’s your fix, but it would be my next choice.


He replaced the spider . . . those are the injectors on this engine

"Cigroller, if the fuel pump was losing pressure why does it start instantaneously when the engine is cold? "

When everything cools back down the vapor condenses back to liquid. So from a cold start the injectors get pressured up with liquid gasoline rather than vapor or a liquid/vapor mix. Low or no fuel pressure also does not mean “empty.”

Also there is “instantaneous” and then there’s “instantaneous.” I actually have this issue in more minor form on a car of mine, so I’ll tell you how mine goes. During the summer on hot days if I park it hot and start it back up within the next hour or so it runs very rough - like it wants to stall out. This past summer it was getting so bad that I decided I’d probably put a new pump in before this summer - i.e., I’m headed for a no-start condition. I also know that if I sit and prime the pump about 10 times - 1) I can actually hear gurgling in my gas tank (under my back seat so its easy) as stuff gets pushed down the return line and 2) the car will then start and run smoothly after that.

When things are cold (car or weather), no problem. I have, in fact, put a pressure gauge on it when everything is hot and when everything is cold. When hot the pressure doesn’t hold. When cold it does. I do know, however, that even if I shut down cold the pressure doesn’t hold overnight. By morning it is zero. But the car starts up without any problem whatsoever with no rough running. But it does take a few more revs on the starter as compared to if I, for example, do a restart during cold weather. When I say a few I just mean literally RR RR RR Roar. It’s not “hard starting.” But if it’s been running - OR if I prime up the pressure first - it is more like RRoar. That’s instantaneous, but it doesn’t feel much different either way.

Anyway - I have no idea if this is your problem. But it’s so easy to check, why not? If you want to, the next time you’re in a situation where it’s likely to do it, prime up the pump about 10 times (run-off-run-off-run-off…) Or ask your mechanic to see what happens to the fuel pressure on a hot shut down.

Of course, the other possibility is what has also been suggested above - hot injectors leaking and flooding the cylinders. For that you try starting with the pedal floored - which is a flood clear. You would also probably see signs of it in your oil after about miles. (Excess fuel smell)/contamination).

A little confused as to what the original problem was. Has the hesitation issue been solved by replacing the fuel pump then? I assume you mean it hesitated when driving, during acceleration right? So as I understand it, the new fuel pump fixed the hesitation during acceleration issue, but now there’s another problem, it won’t start up as quickly it used to on hot starts? i.e. it takes too much cranking time on hot starts. Is that all correct?

If so, there’s several things could cause it. Ignition system components can fail when hot, but work fine when cold. Fuel system problems can cause it too. In addition to measuring the pump pressure, as a initial step in the diagnosis the fuel pressure at the fuel rail should be measured too. The link below might be helpful.

Coolant temperature sensor?

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We have the same problem with the 2002 S 10 four-wheel-drive four-door I had a mechanic tell me it was that sounded like the Cadillac converter’s getting clogged up I like the coolant temperature sensor suggestion that makes sense too. Gonna try coolant temp sensor and fuel regulator to start. Since both kinda cheap. Also going to test fuel pressure leak down. To see if fuel is loosing pressure too fast.

A partially clogged up catalytic converter wouldn’t have much influence on starting. Reduced power when pushing the accelerator pedal down hard, yes.

You’ll get better advice here if you describe what’s happening with your vehicle. Don’t assume the problem back in 2014 is the same.

That is 100% correct, but apparently this person installed a catalytic converter from a Cadillac, and having the wrong catalytic converter in this Chevy truck can likely wreak havoc with with his vehicle’s electronic systems.

More likely used the word Cadillac and we should understand the word catalytic was intended.

Yes, I guess that this is possible.