Low RPMs bucking

[MuRCieLaGo]

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?

[Billy Mild]

Have you done a tuneup or changed O2 sensors?

[MuRCieLaGo]

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.

[Billy Mild]

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.

[A26B]

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 :-)

[MuRCieLaGo]

Originally Posted by Billy Mild

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!

Originally Posted by A26B

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.

[XfireZ51]

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.

[LGAFF]

check both map sensor hoses

[MuRCieLaGo]

Originally Posted by XfireZ51

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...

Originally Posted by LGAFF

check both map sensor hoses

Both? There's only one hose there...

[MuRCieLaGo]

Originally Posted by WVZR-1

You've much to consider and it's been since "day one" so ... a pair of pumps, then there's that what's thought to be repaired ECM pin. What terminal on which connector did that turn out to be? I believe there were three possibilities.

-Repaired ECM pin: can't tell you which terminal. When it was unpinned, the car ran like total crap. It was a ground. But like I said, the shop who repaired it is serious and they have a strong reputation, including from myself in the past.

-Pair of pumps: how do you test the pressure?

-Spark plugs: ordered some new plugs, will be receiving them shortly.

-o2 sensor: I'd like to find a way to test them without having to remove them.

-Coil pack/spark plug wires: not sure how to do this, but there must be a way to test them.



Worst than ever today.

Just before it gets fully warmed up, the car pulled OK. Then, it ran like crap again.

I joined a video, in which we don't really see the misfires, but we can feel the sudden power loss.

https://www.youtube.com/watch?v=cQb882XCwK8

For what it's worth, I'm changing the fuel filter tomorrow.

Should I ask a local tuning shop to run the car on a dyno to help me with the problem?