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Discussion Starter #1
Hi,

I've been noticing that if I'm at part throttle I get a weird surging sensation. It's most common is when I'm on the highway, cruising at ~70mph in 5th, but happens when I'm going slower as well. Always part throttle though, maybe 30% throttle position.

It's as if I were slowly (~1Hz) moving my foot up and down on the gas pedal. This is with a MTX car, so it's nothing with the transmission.

I haven't done much work on this car engine-wise, but I'm curious to know if anyone has had this problem and solved it, or has any ideas on where to start.

Thanks,

Charlie
 

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2006 ZX5 2.0, 2004 ZX3 SVT EE. 2004 ZX3 SVT
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I would pull your spark plugs and have a look as well. I assume your air filter has been changed?
 

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Discussion Starter #5
Did spark plugs a while back, air filter looked good when I checked but hasn't been changed.
 

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Did you check for DTCs?

Paul
 

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Some have complained about overactive EGR doing it. Yank the plug and see.

Shut off a/c and see if it stops, sometimes the compressor kicking on and off can do it.
 

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Discussion Starter #8
Did you check for DTCs?

Paul
Haven't yet, good idea though. I usually don't do this unless I see the dashboard light on. I'll run out and do that this evening.

Some have complained about overactive EGR doing it. Yank the plug and see.

Shut off a/c and see if it stops, sometimes the compressor kicking on and off can do it.
Definitely not the A/C, although maybe a clue is that this engine also stumbles at idle with the A/C on. Extra load and somehow it doesn't compensate for it.
 

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Discussion Starter #9
OK so no codes, just a DTC for a left blinker circuit, weirdly enough. It's working fine, so not sure what was up with that.
 

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Discussion Starter #11
Does it feel like a misfire?
How long since new plugs?
Have you ever replaced the coils?

Paul
I'll be honest, I don't know what a misfire feels like. Would it make sense that it only happens occasionally, and only at a partial throttle opening? If I let off, or step harder on the throttle, the surging feeling goes away...

I replaced the plugs about 8k ago, but the problem definitely happened before that. Never replaced the coils, but I have only owned the car since 156k.
 

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I'm gonna qualify this by admitting that I've never experienced what I'm about to tell you to be an actual fault in any vehicles I've worked on, nor am I that familiar with earlier Foci like yours...but...
In troubleshooting problems similar to yours in other vehicles, I've come across suggestions that in higher mileage vehicles like yours, sometimes the Throttle Position Sensor can get a little worn out in a certain area within its potentiometer (measuring pedal position). That can cause something of a dead spot or dirty/inconsistent signal to the ECU wherein the ECU thinks you're feathering the pedal when you're not. Easy enough to check as I recall. Use a voltmeter to measure the resistance change as you open and close the throttle and see if the change is consistent.
Just tossing it out as an idea.
 

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BTDT with TPS. They commonly show at idle since a lot of time spent there and at the time the engine is vibrating enough to wear the carbon track out in the switch. In my case the engine quit idling right and would just die outright.

'...and only at a partial throttle opening? If I let off, or step harder on the throttle, the surging feeling goes away...'

The EGR cuts off at heavier throttle loads. It can't be on toward idle either. or throttle off, it creates misfire. Part throttle like cruise is where it gets used the most.

That perfectly screams EGR, do a search for postings by a guy here named Marde, he dealt with it.
 

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Discussion Starter #15
In troubleshooting problems similar to yours in other vehicles, I've come across suggestions that in higher mileage vehicles like yours, sometimes the Throttle Position Sensor can get a little worn out in a certain area
...
Easy enough to check as I recall. Use a voltmeter to measure the resistance change as you open and close the throttle and see if the change is consistent.
I can take a look at the TPS through OBD2. That's a bit easier than getting a DMM on the TPS leads.

BTDT with TPS. They commonly show at idle since a lot of time spent there and at the time the engine is vibrating enough to wear the carbon track out in the switch. In my case the engine quit idling right and would just die outright.

'...and only at a partial throttle opening? If I let off, or step harder on the throttle, the surging feeling goes away...'

The EGR cuts off at heavier throttle loads. It can't be on toward idle either. or throttle off, it creates misfire. Part throttle like cruise is where it gets used the most.

That perfectly screams EGR, do a search for postings by a guy here named Marde, he dealt with it.
Awesome, thanks for the tip. Guess I'll have to pull the EGR and take a look.
 

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If it's too aggressive as claimed you won't see anything as it is working 'fine'. New engines accept the EGR amounts as fed, older ones with more mileage sometimes get b-tchy about that. Had a Tempo once that did it specifically at slight cruise to show up on the emission driving test on chassis dyno, it misfired on coming on on older engine to fail me on HC count. On that older less evolved system I took an allen set screw of the right size to plug vacuum hose to EGR valve and drilled a restrictor hole in it to let the EGR come on more gradual, car then passed emissions fine.

It's another complaint like the older engines not idling high enough with a/c on that Ford should create an 'older engine' computer software update for but no, they want to sell you a new car instead.
 

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Discussion Starter #17
So the TPS was fine. Just used an OBD2 reader app and slowly stepped on the pedal and let off, and the TPS output moved smoothly between 19% and 94%. I assume that's just the pot not running all the way to its limits.

Haven't gotten around to really diving into this, but I'll update when I do.
 
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