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“Pulsing” rpm while driving

Mat98caprice

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Hi all!

I have searched and searched and can’t find anyone having a similar issue..
It’s hard to explain but I’ll give it my best shot.. about a week ago I was merging onto a freeway, accelerated to 100km then took my foot off the accelerator and the rpm started pulsing. I could also hear the motor pulsing in line.. speed didn’t change, no shudder... this has happened to me a couple of times under different circumstances.
The first time this happened the AC was running although the other time the AC was off.
Both times I was using cruise control although it still happens when cruise is disengaged..
The car is dual fuel and both times I was using petrol, haven’t had this happen using gas yet.
The second time this happened it was only slightly but the first time the “pulsing” was erratic enough to make my heart race a bit in fear the car was gonna die in the middle of the freeway lol!

Apologies in advance if this has been covered already... if so if someone could point me to the thread would be much appreciated.
If not then any thoughts or suggestions would also be appreciated :)
 

CCCMikey

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Check your transmission fluid. Mine did something similar where it couldn't maintain the TCC Lockup, but only during or briefly after a long climb. I had the transmission serviced after that, and it went away.
 

EYY

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Totally normal. Injectors cut out on overrun. A little fuel is added to ensure the engine stays running. This is what you're hearing.

It won't do it on lpg with a mixer system as the petrol pcm cannot control what the lpg system does.

Most people only notice it with an aftermarket exhaust as it's audibly noticeable.
 

Mat98caprice

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Totally normal. Injectors cut out on overrun. A little fuel is added to ensure the engine stays running. This is what you're hearing.

It won't do it on lpg with a mixer system as the petrol pcm cannot control what the lpg system does.

Most people only notice it with an aftermarket exhaust as it's audibly noticeable.


It’s interesting you say that about the aftermarket exhaust.. the exhaust is very loud for a V6.. a couple of people I’ve spoken to reckon that the Caprice has a louder exhaust and it isn’t aftermarket.. I only got the car about a month ago from a second hand dealer so I have no idea about the previous owners..

What was also concerning to me is when it happened, rpm on the dash was bouncing between 1000-1500rpm while the motor was “pulsing”
 

Mat98caprice

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Check your transmission fluid. Mine did something similar where it couldn't maintain the TCC Lockup, but only during or briefly after a long climb. I had the transmission serviced after that, and it went away.

I will definitely do that! Thanks for your input :)

I also forgot to mention in my original post that the first time it happened while the A/C was running the aircon fan slowed down and stopped pumping air...

I feel like there is a few things I’m gonna need to check out lol
 

EYY

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Stock exhaust is silent. Factory, they're a single 2.25 pipe which splits into 2 mufflers which then goes back to single and through the large rear resi.

Most have rusted out by now - yours is almost certainly aftermarket. The bouncing tach is normal.
 

Mat98caprice

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Stock exhaust is silent. Factory, they're a single 2.25 pipe which splits into 2 mufflers which then goes back to single and through the large rear resi.

Most have rusted out by now - yours is almost certainly aftermarket. The bouncing tach is normal.

Cool thanks heaps for your help :)
 

CCCMikey

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I also forgot to mention in my original post that the first time it happened while the A/C was running the aircon fan slowed down and stopped pumping air...l

Two thoughts there. Could be a vacuum leak somewhere which will cause the aircon system to switch to blowing on your feet at points where vacuum is low (low revs, high throttle position). The aircon will also turn off near full throttle to give max power to the wheels instead.

All guesses of course :)
 

Mat98caprice

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Two thoughts there. Could be a vacuum leak somewhere which will cause the aircon system to switch to blowing on your feet at points where vacuum is low (low revs, high throttle position). The aircon will also turn off near full throttle to give max power to the wheels instead.

All guesses of course :)

Thanks for your input mate :)
Everyone’s help is much appreciated
 
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