Jerk when letting off gas

[the_wuggles]

Hello all!

I have a 1993 Grand Prix 3.1 V6, with 185k. Recently I started having the issue when I let off the gas while at a good speed (60mph you really feel it) it jerks, then drives on fine. Giving it gas - does just fine and speeds right up. Sometimes when putting it into reverse, it acts like it will stall, but then does fine. Today while driving at 60, I let off, it jerked pretty good (but stayed driving fine afterwards) and the service engine light came on. I got to the store, turned it off, in and out then started it back up with no service engine light, but still the jerking.

 

It just had gas in the air intake - and was a bad fuel regulator under the intake. Replaced that and air filter, new gasket. Before that and after, it had an odd idle - so I replaced the IAC valve. Same thing happening - and cleaned out some of the passageways in the throttle body. I'm at a loss - and didn't find anything on a search of the forum. I searched the net and it showed those two things (IAC, fuel regulator) and I just got a throttle position sensor to try tomorrow. I could be way off but at a loss.

 

Any ideas would be appreciated!

 

 

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[RobertISaar]

don't suppose you've taken a look at the engine and trans mounts lately? they won't cause your SES, but as they wear down, any kind of engine reaction is amplified the further gone they are.

[Imp558]

TCC sticking?

[Galaxie500XL]

You really need to know what is causing the check engine light to come on...the codes are your friend in finding the problem...

 

But, at first blush, I have to wonder if the jerk, and the nearly dying issue is a torque converter clutch issue.

[the_wuggles]

Thank you everyone! The mounts don't look terribly bad but are a bit loose, I'll replace those for the $10 each.

 

The code I can't read - the 1993 doesn't use the obd2, so auto part stores and my reader is out. It'd be $75 at my auto repair guy.

 

If it is the torque convert clutch - would that mean just a new solenoid? Or transmission work?

 

 

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[Nas Escobar]

If it's 93, jump the 2 top right pins with a paper clip and the SES light will blink the code. Then google the code and you'll know what it is. 

[vipmiller803]

Check the code like Nas recommended. Also, check the trans modulator and its vacuum connections. Those can leak trans fluid into the intake. Also check tps since those seem to give trouble if you look at them wrong and can easily cause an SES

[Galaxie500XL]

I didn't even think of the modulator...a leaking or broken vacuum line to the modulator can cause interesting behavior.  Fortunately, they're easy to replace, and cheap...around $10.00.

[Imp558]

If it's 93, jump the 2 top right pins with a paper clip and the SES light will blink the code. Then google the code and you'll know what it is.

Pin A to Pin B, I bet Miller just solved it though, didn't even think about it being a 4t60.

[the_wuggles]

Ok tomorrow I will try the ses light by jumping the top 2 right terminals. Will it still show even if it is not active? It only came on the once and hasn't stayed on. I found a broken vacuum line from the very top of the transmission that went to a piece of metal by the battery? Maybe 1.5' long or so, went to a hanger type metal (not hollow). Will take a picture tomorrow. Is there a way to test if the modulator is bad if that is it? Or just guessing game? I do have a new throttle position sensor - should I try it or no? Will do more stuff tomorrow after reading the responses, get a pic of that vacuum line, and return back!

Thanks again everyone!!

 

 

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[Galaxie500XL]

Based on your description, it's hard to know for certain if it's a hose going to the modulator, or to the vacuum sphere, which is under the battery...but most likely, it's to the vacuum sphere.  Classic symptoms are you only have defroster vents when this is disconnected, but a large vacuum leak could definitely cause the symptoms you're describing.

[vipmiller803]

Should be stored for a few cycles, so if you read it soon it should pop up.

Will it still show even if it is not active? It only came on the once and hasn't stayed on.

 

 

Not a guessing game. The modulator is really simple. You test it like any other diaphragm. If it is not leaking fluid into the intake, best way to test it would be to remove it, push the plunger that goes in the trans, cap the vacuum line with a finger, and see if the plunger springs back while the vacuum connection is capped. Should not spring back until you remove your finger.

Is there a way to test if the modulator is bad if that is it? Or just guessing game?

It looks like you are in our wonderfully corrosive state. The vacuum side may have just rusted though.

 

 

Up to you on the TPS. I wouldn't until you read those codes.

Edited March 10, 2017 by vipmiller803

[Imp558]

If you found a broken vacuum line regardless you have to fix the system. A transmission with a vacuum modulator will slam pretty hard without a vacuum source. The vacuum sphere is a reservoir that is living in the fender it is in the neighborhood of the marker light on the driver side theoretically.

[Galaxie500XL]

If you were wondering, the modulator controls (or by definition modulates), line pressure.  Without a properly functioning modulator (or a severe enough vacuum leak), line pressure stays maxed all the time, which is why shifts can be so jarring.

[Nas Escobar]

Yeah it will show the code if it's store

[the_wuggles]

Thanks everyone. - I am working on it right now. Got a picture of that vacuum line..

 

 

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[the_wuggles]

Ok. So, to start, I fixed the vacuum line. It helped with idling and ran a bit better but not entirely. It was -3 with windchill today so I'm lucky I did that much. I don't have a garage or anywhere to stick the darn thing.

 

Any-who.. the SES blinked 12. According to Haynes (if I read correctly) it means nada. And I let it blink more than the 3 times, left it in for 30 seconds at least with it only flashing 12.

 

Any thoughts or suggestions? It still bucks, but not every time I let off the throttle now. I checked the torque strut mounts and those are bad - I'm ordering them (no one has them anywhere close to me), after I post this.

[Imp558]

On the right track bro, fix the obvious then see where the chips fall

[Nas Escobar]

Ok. So, to start, I fixed the vacuum line. It helped with idling and ran a bit better but not entirely. It was -3 with windchill today so I'm lucky I did that much. I don't have a garage or anywhere to stick the darn thing.

 

Any-who.. the SES blinked 12. According to Haynes (if I read correctly) it means nada. And I let it blink more than the 3 times, left it in for 30 seconds at least with it only flashing 12.

 

Any thoughts or suggestions? It still bucks, but not every time I let off the throttle now. I checked the torque strut mounts and those are bad - I'm ordering them (no one has them anywhere close to me), after I post this.

I'd check the whole vacuum system to verify everything is in order. I personally had lines crossed in my 93 Z. As far as the codes go it's blink 12 for 3 times and then you'll get the codes. If you only got 12s after the 3rd blink then you'll have to wait until the codes are thrown again or read them with an ALDL cable and tuner pro

[the_wuggles]

Yeah - I just got a bunch of 12s, next time when it happens I will know what to do at least. If I'm correct - there's a diagram under the hood of vacuum lines? Hope that's correct otherwise I am so in over my head.

 

 

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[Nas Escobar]

Yup there is.

 

If they're crossed the easiest way to sort em is to get a piece of paper and draw out the vacuum lines till you end up with a copy of the diagram in the hood.

[the_wuggles]

Well, I found a small vacuum leak - just a coped end broke so I moved the cap end forward the half inch it broke.

 

Seeing it from the engine - I pull the throttle, it starts to die for a split second then picks back up. The bucking seems mostly gone? The rest I think just the bad torque mounts. Any other ideas now that it's changed a bit?

 

 

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[Imp558]

Lower engine mounts, there's one on the front of the engine and one on the tranny. They are likely bad unless you know they've been replaced.

[the_wuggles]

Okay everyone - sorry I've been out for a while, family had a horrible bout of stomach flu so that took me down for a couple weeks.

 

Ok, I unhooked the torque converter - that's not the issue, it still has the problem. I also installed the TPS sensor, and the gas pedal is more sensitive now, but no go. After all this, it has flipped. It's now when your press the gas it starts to die, then picks up.

 

So far replaced:

Fuel regulator

Air filter

TPS sensor

IAC valve

Torque strut mounts

Spark plugs

Wireset

 

I am at a complete loss still, and any help again would be appreciated.

 

 

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[Imp558]

Still need to check for leaks at the vacuum ball. It's less than amusing to get to.