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Old 05-09-2016   #1
MuRCieLaGo
 
Join Date: Aug 2014
Location: Trois-Rivieres (QC)
Posts: 473
Default Low RPMs bucking

Hi,

'91 ZR-1.

Valet key always ON.

I'm getting severe buckings at WOT when I'm under 3,500 RPM. High RPM is fine, pulls strong.

For 3 or 4 miles this morning the car felt like it was running on 7 cylinders, from 1,000 RPM to 7,000 RPM. I took a 400 miles ride and it never happened again. Fuel consumption was good.

However, the low RPMs bucking is still happening. It's been like this since I bought the car 3,000 miles ago. Warm or not.

New from this year though: sometimes, the RPM needle would jump from 4,000 RPM to 1,000 RPM suddenly only for a moment (total power loss), then immediately comes back to 4,000 RPM. Happens especially when the car isn't fully warmed up yet.

No SES light.

I electrically tested the injectors last fall and everything was perfect.

You guys got an idea? Is it normal to get bucking at low RPMs?

Last edited by MuRCieLaGo; 05-09-2016 at 12:24 PM.
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Old 05-09-2016   #2
Billy Mild
 
Join Date: Aug 2014
Location: Kansas City, MO
Posts: 838
Default Re: Low RPMs bucking

Have you done a tuneup or changed O2 sensors?
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Old 05-09-2016   #3
MuRCieLaGo
 
Join Date: Aug 2014
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Default Re: Low RPMs bucking

I recently changed the headers and added a o2 sensor 2 feet extension on passenger side. I also had Marc Haibeck burn me a chip.
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Old 05-09-2016   #4
Billy Mild
 
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Default Re: Low RPMs bucking

The bucking has not changed at all?

I would at least throw a set of new spark plugs in it. If that doesn't help then start looking at coils and wires. I have a similar issue except my car isn't always noticeable.
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Old 05-09-2016   #5
A26B
 
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Default Re: Low RPMs bucking

1. Sounds like a closed loop issue
2. You recently altered O2 sensor wiring
3. Bucking you describe can be directly associated with a single bank, shorted O2 sensor
4. Shorted O2 sensor symptoms will disappear in open loop, i.e. above 3200rpm

Recommend you check there first. Verify extension is properly wired, & make sure the connections are dry inside, no moisture. Do not tie back the extension underneath the AC condensate drain. Ask me how I know :-)
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Old 05-09-2016   #6
MuRCieLaGo
 
Join Date: Aug 2014
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Posts: 473
Default Re: Low RPMs bucking

Quote:
Originally Posted by Billy Mild View Post
The bucking has not changed at all?

I would at least throw a set of new spark plugs in it. If that doesn't help then start looking at coils and wires. I have a similar issue except my car isn't always noticeable.
Just ordered a 5/8'' socket with a swivel to change the plugs!

Quote:
Originally Posted by A26B View Post
1. Sounds like a closed loop issue
2. You recently altered O2 sensor wiring
3. Bucking you describe can be directly associated with a single bank, shorted O2 sensor
4. Shorted O2 sensor symptoms will disappear in open loop, i.e. above 3200rpm

Recommend you check there first. Verify extension is properly wired, & make sure the connections are dry inside, no moisture. Do not tie back the extension underneath the AC condensate drain. Ask me how I know :-)
I'm a bit surprised by what you are saying there. I hope there's a way to test the sensors without having to remove them from the headers... I'm pretty sure the wiring is fine...

Like I said, no SES light. I check the codes even if I don't have a SES light and no code is stored.
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Old 05-09-2016   #7
XfireZ51
 
Join Date: May 2007
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Default Re: Low RPMs bucking

Keep your foot in it and continue the bucking. See if it sets a code. You may see a Code 61 and a Code 44. The bucking may be due to a bad fuel pump.
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Old 05-10-2016   #8
LGAFF
 
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Default Re: Low RPMs bucking

check both map sensor hoses
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Old 05-10-2016   #9
MuRCieLaGo
 
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Default Re: Low RPMs bucking

Quote:
Originally Posted by XfireZ51 View Post
Keep your foot in it and continue the bucking. See if it sets a code. You may see a Code 61 and a Code 44. The bucking may be due to a bad fuel pump.
I already tried this and never saw any SES light... The fuel pump would be a good guess though, previous owner told me some water went in the fuel tank and it froze. I tested the pump last fall and the resistance was a bit too high...

Quote:
Originally Posted by LGAFF View Post
check both map sensor hoses
Both? There's only one hose there...
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Old 05-10-2016   #10
LGAFF
 
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Default Re: Low RPMs bucking

one under the ecm
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