hesitation, low power

urbex · 10-30-2009, 07:46 PM

'91 S10 Blazer, 4.3L, auto, ~122,000miles.

Was running a bit off when I got the truck, so I don't have a good baseline to work off of, but judging from the last S-truck I had ('91 Sonoma ext cab, 4.3L, auto, about 100,000 miles), this one has noticeably less power (feel like I'm driving a 4cyl half the time), and throttle response lags big time.

It occasionally has a hunting idle, especially when cold. Usually ranges from 550-1200 rpm, and is a steady, slow up and down. Occasionally has no power at all when accelerating - _anything_ above idle seems to be an extraordinary accomplishment when that happens, usually is OK up to about 40 or 50 percent throttle. From roughly 40% 100% seems to be a dead zone. If I mat it from a stop, you can hear the intake open up, but it's like it's taking 2-3 seconds to spool up, then starts moving. WOT does not cause the engine to die, just take a while before responding. When it does start moving, it seems smooth, but doesn't feel as fast as it seems like it should. It does this whether driving or running in the driveway. When in the driveway, it's also stuttering while it's revving up.

Things done in the past week: (scan tool is WinALDL running on my laptop)

New AC Delco spark plugs, gapped to .035"
TPS tested both with a volt meter and scan tool, tests good. Swapped for a new one too, just in case.
All rubber vacuum lines replaced under hood
MAP sensor shows proper operation with scan tool
Injectors appear to be spraying clean, and in the proper cone pattern. When goosing the throttle, it's also obvious that there is more fuel being delivered, even while the engine is hesitating.
Cap/rotor cleaned terminals cleaned inside/out.
EGR cleaned, new gasket.
IAC cleaned, new gasket
New air filter.

I'm about to go out to buy a fuel pressure gauge, and check that as well. Although given that fuel flow is increasing, when the throttle opens up, I'm not sure that's the problem.

Any ideas?

urbex · 11-01-2009, 09:40 PM

Update: I'm fairly positive it is the fuel pump. Pulled the engine to frame line connection on the supply side, hooked up a gauge inline, and got about 5 psi with the truck running. Pinching off the return line had no effect. To rule out the fuel filter, I also tested a dead head connection off the fuel filter supply side, again only getting about 5psi. when I disconnected the gauge, and just let the pump free flow, it barely was dribbling out.

Frankly, I'm amazed the truck even ran at all like this. With it being able to do 70 on the freeway, I didn't think low fuel pressure would be to blame. I picked up another pump today, and will be installing it tomorrow morning/afternoon most likely. I'll report back after.

urbex · 11-02-2009, 09:08 PM

Yep, fuel pump was the problem. New one went in today, along with new filter strainer and fuel filter, and the hesitation is gone. Still seems like it's idling a bit rough, but doesn't sound like it's missing. I'm still going to change out the cap/rotor and plug wires here in a few days anyways, just to make sure.

03blazer · 11-03-2009, 12:47 AM

Probably wouldn't hurt to open the gap on the plugs a little.

urbex · 11-03-2009, 03:03 PM

I was thinking about that, actually. Perhaps bumping up to .040 or even .045. I ran my RPO codes last night too, and found that this truck has a 3.08 gear in it, compared to my Sonoma having had a 3.42, which given the lighter weight of the pickup combined with the lower gears would certainly explain the seemingly much better performance.

10-30-2009, 07:46 PM	#1
urbex Registered User Age: 29 Join Date: Oct 2009 Posts: 17 Location: Glendale, AZ User is: OffLine	hesitation, low power '91 S10 Blazer, 4.3L, auto, ~122,000miles. Was running a bit off when I got the truck, so I don't have a good baseline to work off of, but judging from the last S-truck I had ('91 Sonoma ext cab, 4.3L, auto, about 100,000 miles), this one has noticeably less power (feel like I'm driving a 4cyl half the time), and throttle response lags big time. It occasionally has a hunting idle, especially when cold. Usually ranges from 550-1200 rpm, and is a steady, slow up and down. Occasionally has no power at all when accelerating - _anything_ above idle seems to be an extraordinary accomplishment when that happens, usually is OK up to about 40 or 50 percent throttle. From roughly 40% 100% seems to be a dead zone. If I mat it from a stop, you can hear the intake open up, but it's like it's taking 2-3 seconds to spool up, then starts moving. WOT does not cause the engine to die, just take a while before responding. When it does start moving, it seems smooth, but doesn't feel as fast as it seems like it should. It does this whether driving or running in the driveway. When in the driveway, it's also stuttering while it's revving up. Things done in the past week: (scan tool is WinALDL running on my laptop) New AC Delco spark plugs, gapped to .035" TPS tested both with a volt meter and scan tool, tests good. Swapped for a new one too, just in case. All rubber vacuum lines replaced under hood MAP sensor shows proper operation with scan tool Injectors appear to be spraying clean, and in the proper cone pattern. When goosing the throttle, it's also obvious that there is more fuel being delivered, even while the engine is hesitating. Cap/rotor cleaned terminals cleaned inside/out. EGR cleaned, new gasket. IAC cleaned, new gasket New air filter. I'm about to go out to buy a fuel pressure gauge, and check that as well. Although given that fuel flow is increasing, when the throttle opens up, I'm not sure that's the problem. Any ideas?
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11-01-2009, 09:40 PM	#2
urbex Registered User Age: 29 Join Date: Oct 2009 Posts: 17 Location: Glendale, AZ User is: OffLine	Re: hesitation, low power Update: I'm fairly positive it is the fuel pump. Pulled the engine to frame line connection on the supply side, hooked up a gauge inline, and got about 5 psi with the truck running. Pinching off the return line had no effect. To rule out the fuel filter, I also tested a dead head connection off the fuel filter supply side, again only getting about 5psi. when I disconnected the gauge, and just let the pump free flow, it barely was dribbling out. Frankly, I'm amazed the truck even ran at all like this. With it being able to do 70 on the freeway, I didn't think low fuel pressure would be to blame. I picked up another pump today, and will be installing it tomorrow morning/afternoon most likely. I'll report back after.
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11-02-2009, 09:08 PM	#3
urbex Registered User Age: 29 Join Date: Oct 2009 Posts: 17 Location: Glendale, AZ User is: OffLine	Re: hesitation, low power Yep, fuel pump was the problem. New one went in today, along with new filter strainer and fuel filter, and the hesitation is gone. Still seems like it's idling a bit rough, but doesn't sound like it's missing. I'm still going to change out the cap/rotor and plug wires here in a few days anyways, just to make sure.
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11-03-2009, 12:47 AM	#4
03blazer Gettin' ready for V8! Join Date: Dec 2007 Posts: 338 Location: Gulfport, MS User is: OffLine	Re: hesitation, low power Probably wouldn't hurt to open the gap on the plugs a little.
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11-03-2009, 03:03 PM	#5
urbex Registered User Age: 29 Join Date: Oct 2009 Posts: 17 Location: Glendale, AZ User is: OffLine	Re: hesitation, low power I was thinking about that, actually. Perhaps bumping up to .040 or even .045. I ran my RPO codes last night too, and found that this truck has a 3.08 gear in it, compared to my Sonoma having had a 3.42, which given the lighter weight of the pickup combined with the lower gears would certainly explain the seemingly much better performance.
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