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2000 4.3 rough idle & hesitation

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Old 05-27-2010, 01:42 PM   #1
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2000 4.3 rough idle & hesitation

This is a problem with my 2000 Blazer ZR2:

This just cropped up this morning while leaving for work - the idle is very rough and sits about 500 rpm with the AC off. Acceleration is sluggish, pushing on the pedal gives no immediate response. The engine didn't try to stall or feel like it bogged down when I hit the gas, it just doesn't seem to react without a delay. Fuel level is between 1/4 and 1/2 tank. I had to sit at a railroad crossing for a few minutes, the idle felt rougher than my 91 with its Edelbrock cam. I ended up taking it back to the house and driving the Malibu instead. These things always seem to happen when you're already running late!

I'm going to pick up some fuel treatment tonight and try to get it to the gas station to top off the tank - maybe it's bad gas. The past time I drove it was on Sunday and it ran normally - mix of city and highway on that trip. I'm not sure what's changed since then.

So, what should I look at next if the issue doesn't clear up? I'm feeling rusty on my diagnostics lately. Plugs are about two years old, air filter around last October, replaced the charcoal canister and both valves last winter. Alternator was tested earlier this year when I replaced the battery.

Last edited by Matt 4.3 TBI; 05-27-2010 at 01:44 PM.
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Old 05-27-2010, 02:58 PM   #2
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Re: 2000 4.3 rough idle & hesitation

The normal suspects, vacuum leaks, fuel filter, fuel pressure, cap, wires, rotor, etc.
Old 05-27-2010, 06:45 PM   #3
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Re: 2000 4.3 rough idle & hesitation

Make sure that a plug wire hasn't worked loose.
What does the exhuast look, smell, and sound like when it is having these issues?
I'd highly recommend connecting a vacuum gauge and running some simple tests before making expensive gusses.
Also, pull the dip stick and run the oil between your fingers. How does it feel? How does it smell?
When it's acting up; pull the oil cap. How does it sound? Place your hand over the hole. What do you feel?
Is you PCV valve plugged? is it still working? Smell funny?

Good luck.

Last edited by Bill Wheats; 05-27-2010 at 06:50 PM.
Old 05-27-2010, 10:35 PM   #4
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Re: 2000 4.3 rough idle & hesitation

Quote: Originally Posted by Bill Wheats
Make sure that a plug wire hasn't worked loose.
What does the exhuast look, smell, and sound like when it is having these issues?
I'd highly recommend connecting a vacuum gauge and running some simple tests before making expensive gusses.
Also, pull the dip stick and run the oil between your fingers. How does it feel? How does it smell?
When it's acting up; pull the oil cap. How does it sound? Place your hand over the hole. What do you feel?
Is you PCV valve plugged? is it still working? Smell funny?

Good luck.
It definitely sounds and feels like a misfire. Took it down the highway to the gas station and back. When cruising it is resistant to acceleration until it downshifts. Still accelerates well under hard throttle. Doesn't sputter and try to stall when snapping the gas open from a stop or letting off the gas quick from full throttle.

Plug wires appear to be in place properly. I attached my timing light to each wire as well - each one caused the light to blink in about the same pattern. So, distributor and cap/rotor appear to be good. Very quick strobe when the light is attached to the coil wire.

Exhaust has a strong gas smell - normally no odor at all, like smelling a running hair dryer. Tailpipe shakes noticeably, distinct 'chugging' sound as it idles. Engine visibly shakes under the hood.

Vacuum read 18 in/Hg but I could only find ported vacuum so it didn't act right when I tried snapping the throttle.

PCV valve rattles. You can feel the stopper snap back and forth by covering and opening the hole with your finger while the engine is running.

Oil is relatively fresh, only about 300 miles since the last change. Light amber color, no visible chunks and doesn't feel gritty. Didn't feel any pressure coming from the PCV valve port in the DS valve cover.

Still no MIL codes.

Last edited by Matt 4.3 TBI; 05-27-2010 at 10:38 PM.
Old 05-28-2010, 09:22 AM   #5
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Re: 2000 4.3 rough idle & hesitation

A code finally-
P0155: O2 sensor heater circut malfunction (bank 2 sensor 1).

So which O2 is this one? Passenger side before the cat?
Old 05-28-2010, 09:48 PM   #6
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Re: 2000 4.3 rough idle & hesitation

Installed the new O2 and cleared the code. So far no difference. Still chugging and shaking at idle, exhaust smells like gas and bad hesitation when you hit the gas. Surging seems even more noticeable now when holding the throttle steady.
Old 05-28-2010, 10:23 PM   #7
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Re: 2000 4.3 rough idle & hesitation

A bit more info-

I don't feel comfortable trying to do a full ODBII drive cycle - I really don't want to take it on the highway to get it up to the required speeds. I started it up and let it idle in D with the AC and rear defroster on for over two minutes - I let it go until it reached operating temp. It was idling noticeably rough as expected. I then put it in P and gave part throttle to get the RPM's up to 3000 - no audible misfires, backfires or sputtering. I let off the throttle and then shifted back into D and let it idle. Now it was idling extremely rough, shaking the vehicle visibly. I let it keep idling like this (RPM's about 500). It sounded like it was straining to keep running. Sooty smoke was visible coming from the exhaust - along with the gas smell. Obviously there's a very rich condition going on here but I'm not sure what to check. No codes yet. I'll try this one more time once it cools off so it can do a full warmup cycle.

It still starts up with no trouble so it doesn't seem like the fuel pressure regulator is leaking. I haven't got any misfire codes so I don't think the poppets are sticking (but I don't really know all the symptoms).

Unfortunately I called the local shop to see if they could lend a hand - they have no one on staff to do diagnostics until after the holiday weekend.

As I mentioned, I have the MFI spider kit but I've been avoiding the chore of installing if until a problem arose with the CSFI injectors.

Any other investigation I can do to help narrow this down?
Old 05-28-2010, 11:07 PM   #8
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Re: 2000 4.3 rough idle & hesitation

I went back out and idled it in drive and park again to see how it acted.

It was running rough but staying steady as it did before - until I put it in neutral and brought the RPM's up to 2500. Once I let it back down to idle it started bucking and sputtering again - even worse in drive. I let it idle to see if it would actually stall - the computer actually idled it up to about 1500 without warning for about five seconds, then let it back down. It did this once more in the next five minutes.

It was nearly stalling when I shut it off and it exhibited another strange symptom I thought I imagined the last time - dieseling. It sputtered and ran a few more seconds after I turned the key to the off position.
Old 05-29-2010, 12:13 AM   #9
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Re: 2000 4.3 rough idle & hesitation

Additional:

After this it's hard to restart - like when I had a leaking fuel pressure regulator. I'm starting to wonder if the injectors, lines or poppets are leaking fuel inside the plenum
Old 05-29-2010, 01:01 AM   #10
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Re: 2000 4.3 rough idle & hesitation

I hate to say it but you need to go back to the drawing board and redo your basic troubleshooting.
There is a reason why I recommended doing what I posted. The solution is simple.
And stop replacing O2 sensors until the engine is fixed. Then you can replace them at will.
Old 05-29-2010, 01:17 AM   #11
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Re: 2000 4.3 rough idle & hesitation

You and I may have the same problem with our trucks. What that is however, idk.

Been trying to figure it out for months haha
Old 05-29-2010, 01:40 AM   #12
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Re: 2000 4.3 rough idle & hesitation

Quote: Originally Posted by 01S10
You and I may have the same problem with our trucks. What that is however, idk.

Been trying to figure it out for months haha
You should start a new thread.
Old 05-29-2010, 02:45 PM   #13
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Re: 2000 4.3 rough idle & hesitation

Quote: Originally Posted by Bill Wheats
You should start a new thread.
I already did... but I don't want to spam this guys thread with posts. Just sayin im interested to see how he fixes his problem.
Old 05-29-2010, 03:38 PM   #14
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Re: 2000 4.3 rough idle & hesitation

im interested as well. unfortunately.
Old 06-13-2014, 01:26 AM   #15
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Re: 2000 4.3 rough idle & hesitation

How was this problem fixed? I've got a 2003 s10 Xtreme that is having the same problem. the only codes it has pulled up was for a mass air flow sensor and the drivers side pre cat o2 sensor.
Old 06-13-2014, 01:51 AM   #16
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Re: 2000 4.3 rough idle & hesitation

With these symptoms, I'd guess fuel pressure was low.

If you truck is sluggish and unresponsive, check fuel pressures.
Old 06-13-2014, 09:04 AM   #17
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Re: 2000 4.3 rough idle & hesitation

Quote: Originally Posted by Nathan S.
How was this problem fixed? I've got a 2003 s10 Xtreme that is having the same problem. the only codes it has pulled up was for a mass air flow sensor and the drivers side pre cat o2 sensor.
The problem was a combination of leaking injector poppets and a faulty PCM. The truck has been running perfectly for the past four years since I installed the MPFI injectors and had the PCM replaced.
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