FAQ: Hesitation, Stutter(Studder), Shudder, Stumbling, on Acceleration/Load

ZEP wrote:

Rick, I can't recall the exact code. I always work with the guy in the bay and he showed me the code. The first code I got maybe 1.5 months ago was that the fuel was running lean, and getting a new SC gasket helped fix that problem. But the SES light stayed on, so I took it in again to get it scanned (I can't wait to buy a scanner some time soon), and my mechanic showed me the code. Something like: CPM Sensor Out of Range."

So, they replaced the camshaft position sensor. The light stayed on though even though he cleared it, and the car gave us the same code... CPM Sensor Out of Range. So he doesn't know what to do. He'll check it again some time this week to see if everything looks alright, but he said that he thought the computer might need to be re-flashed.

That's just what he told me... I've heard I may need to have a CASE Learn procedure done, not a re-flash, but I'm not sure, so I was just going to hold out on that until I get the wires done next weekend.

A CASE Learn sounds right. I think there may be mone info on that somewheres on this site. The right Vetronix or OTC tool will run a CASE learn for you.

Albertj

Update:

I finally got the chance to come home today, and I replaced the cheap wires a shop put in the car with new AC Delco wires. I could feel the difference clearly after I drove the car once I was done. As far as I can tell, I no longer have a hesitation problem. The 98 feels like its almost brand new. Thanks for the help everybody!

On another note, I keep getting the code I described lightly earlier and the car is now in the shop for them to check out the cam position sensor, check the timing chain and all that, but that is another issue. Thankfully it doesn't seem to be affecting driveability at this point!

Ok when I accelerate (even just a lil bit) my car hesitates but that's only when it is cold, then when it warms up it runs fine.

The only thing I've done recently was changing the tranny fluid and filter, and then filled the gas tank with a bottle of Lucas fuel injection cleaner. I know i have a problem with my crank sensor which I plan to change it because I had the same problem that "albertj" had that when it gets hot and I shut the engine the car will not start until I let it cool down, but I'm not 100% sure if that is the problem. If you have any suggestions where I can start I'll appreciate it.

Thank you.

When car is idling watch the tachometer. If the needle flutters, it is probably time to pull and clean the throttle body and IAC air passages.On-car cleaning of the TB on these cars does not clean the area behind the throttle plate. Be sure to use "coating-safe" cleaner.

Thanks albertj.

When the car is idling the needle doesn't flutters, but it goes down when I accelerate (hesitation) then when it gains speed it runs fine. I thought it might be a vacuum leak but I looked under the hood real quick and saw no vacuum lines bad as I replaced some of them a few weeks ago. And yes, when I gutted the air box I saw that the TB was very dirty, I'll try to clean it tomorrow as my days off (work) are SUN and MON so I cannot get back with you guys until Tuesday [ no home internet ]

Any other suggestion?

Thank you.

rk0ehn wrote:

Thanks albertj.

When the car is idling the needle doesn't flutters, but it goes down when I accelerate (hesitation) then when it gains speed it runs fine. I thought it might be a vacuum leak but I looked under the hood real quick and saw no vacuum lines bad as I replaced some of them a few weeks ago. And yes, when I gutted the air box I saw that the TB was very dirty, I'll try to clean it tomorrow as my days off (work) are SUN and MON so I cannot get back with you guys until Tuesday [ no home internet ]

Any other suggestion?

Thank you.

No other suggestions right now except to try to find a service manual on eBay and good luck catching up on the maintenance for your car. When you have completed the work you contemplate please post results. also please consider replacing the rest of those old vacuum lines (there are not that many).

Have you ever replaced the spark plug wires? I'd start there, fresh plugs too.

Well I cleaned the TB but it still runs like that but only when it's cold!!, that's the weird thing, I thought it might be a faulty ECT [engine coolant temp] sensor or a MAF? and when I first got the car I pulled the plugs and they looked fine as a normal combustion, I saw that they replaced a coil ack before (number 3-6 I think) but I'm not sure about the cables, I might consider changing them as soon as I get money. I'm considering that a sensor is not working right

albertj wrote:

No other suggestions right now except to try to find a service manual on eBay and good luck catching up on the maintenance for your car.

Yeah! I'll buy it hopefully soon, every car I had before I always repaired them with their respective service manual, but I got a lil bit disappointment when I saw that there was no manual for this car, I'm looking for a service manual in a DVD or something for about 24.95 free shipping 8GB big.

I'll do more diagnosis on the weekend, do you have any info how to check the ECT as far as resistance specs? and the MAF?, the thing is that I have no codes nor any CEL.

* Start up fuel enrichment on fuel injected engines. When the PCM receives a cold signal from the coolant sensor, it increases injector pulse width (on time) to create a richer fuel mixture. This improves idle quality and prevents hesitation while the cold engine is warming up. As the engine approaches normal operating temperature, the PCM leans out the fuel mixture to reduce emissions and fuel consumption. A faulty coolant sensor that always reads cold may cause the fuel control system to run rich, pollute and waste fuel. A coolant sensor that always reads hot may cause cold driveability problems such as stalling, hesitation and rough idle.


http://www.aa1car.com/library/coolant_sensors.htm

This is what it makes me think that it's not working properly

rk0ehn wrote:

Well I cleaned the TB but it still runs like that but only when it's cold!!, that's the weird thing, I thought it might be a faulty ECT [engine coolant temp] sensor or a MAF? and when I first got the car I pulled the plugs and they looked fine as a normal combustion, I saw that they replaced a coil ack before (number 3-6 I think) but I'm not sure about the cables, I might consider changing them as soon as I get money. I'm considering that a sensor is not working right

albertj wrote:

No other suggestions right now except to try to find a service manual on eBay and good luck catching up on the maintenance for your car.

Yeah! I'll buy it hopefully soon, every car I had before I always repaired them with their respective service manual, but I got a lil bit disappointment when I saw that there was no manual for this car, I'm looking for a service manual in a DVD or something for about 24.95 free shipping 8GB big.

I'll do more diagnosis on the weekend, do you have any info how to check the ECT as far as resistance specs? and the MAF?, the thing is that I have no codes nor any CEL.

* Start up fuel enrichment on fuel injected engines. When the PCM receives a cold signal from the coolant sensor, it increases injector pulse width (on time) to create a richer fuel mixture. This improves idle quality and prevents hesitation while the cold engine is warming up. As the engine approaches normal operating temperature, the PCM leans out the fuel mixture to reduce emissions and fuel consumption. A faulty coolant sensor that always reads cold may cause the fuel control system to run rich, pollute and waste fuel. A coolant sensor that always reads hot may cause cold driveability problems such as stalling, hesitation and rough idle.


http://www.aa1car.com/library/coolant_sensors.htm

This is what it makes me think that it's not working properly

might look at what a coolant sensor will cost you compared to the diagnosis (you are not going to find a DVD manual for these cars easily, you may find a paper manual on ebay) and just put one in.

...what I would do: I would disconnect the coolant sensor and measure its resistance with a multimeter while cold and out of circuit, then reconnect it and start the car and warm it up - then turn off car and disconnect and measure again. If the resistance of that sensor did not change I'd get a new coolant sensor without any hesitations. Naturally I would want to know what the reading was supposed to be but without the information I can still measure if it is changing or not with temperature - and if it is not changing then it has to get replaced.

Albertj

Quote :

I saw that they replaced a coil pack before (number 3-6 I think) but I'm not sure about the cables, I might consider changing them as soon as I get money.

If the car has 176k miles, and you're not sure, this should be your next step. The coil pack replacement is a clue that the last owner knew about an ignition problem. Coil packs don't normally wear out, but they will go bad if the wires or plugs are not functioning 100%. You say the plugs look good, so that makes me think even more you need to replace the wires - they probably caused the previous coil to fail.

AA wrote:

Quote :

I saw that they replaced a coil pack before (number 3-6 I think) but I'm not sure about the cables, I might consider changing them as soon as I get money.

If the car has 176k miles, and you're not sure, this should be your next step. The coil pack replacement is a clue that the last owner knew about an ignition problem. Coil packs don't normally wear out, but they will go bad if the wires or plugs are not functioning 100%. You say the plugs look good, so that makes me think even more you need to replace the wires - they probably caused the previous coil to fail.

...consider getting a set of OE (Delco) wires or a set of Magnecors --> http://www.magnecor.com

Thanks guys,

I'll go ahead and measure the resistance change on the ECT and definitely will change the wires as soon as I can, but do you think that the cables will not work properly with temp changes? yesterday when I came out of work and started the engine it was running fine after sitting for 8 hrs, but this morning it was acting up again, I guess after sitting overnight it's when something happens.

albertj wrote:

might look at what a coolant sensor will cost you compared to the diagnosis (you are not going to find a DVD manual for these cars easily, you may find a paper manual on ebay) and just put one in.

well I found one on Ebay and it was kinda interesting.

http://cgi.ebay.com/ebaymotors/BUICK-RIVIERA-1996-1997-1998-1999-REPAIR-SERVICE-MANUAL-/220869875638?pt=Motors_Manuals_Literature&hash=item336cded7b6

LOL, they even lowered the price, I think it's pretty good though.

I'm just waiting to stabilize my economical situation and then do a LOT of maintenance on my car.

Thanks again for the good ideas.

We'll see what you think of that manual after you get and use it. Good luck.

Ok I did test the ECT and this is what I got:

When cold the Black wire and the Yellow wire reads 2,000 ohms
When hot (about 180F) it reads 377 ohms

So, I guess it does work and I can discard that ECT sensor.

Do you think that it could be the O2 sensor? I know the engine stays in open loop when it's cold until it goes into closed loop when the O2 reaches the normal operating temp at about 600F, probably its not getting enough fuel when it's cold... lol...???

I'm not sure what's going on....

Sometimes when I step on the gas pedal when it's a cold start the rev goes down and then right up and starts accelerating, but I'm not sure if that is normal just because the engine is cold.

Have you tried cleaning your MAF sensor?

I did the TB but haven't done the MAF yet I'll do it next paycheck to buy a can of MAF sensor cleaning :S, as I'm living on a really tight budget until the next month.

I think that might be the cause and hopefully fix it with a good and carefully MAF cleaning.

Thanks!

you can test the MAF by just unplugging it and driving around a bunch. (since you have a 96, you can run no problem with no MAF, as the PCM just uses your MAP sensor.) If the symptom goes away, then it either needs cleaning, replacing, or the wire to the PCM has an issue.(all of which might cause an issue like you describe.) The "unplugging" test is more effective than just giving it a clean since it can still pin-point your problem even if its a wire or actually bad MAF(that cleaning wont help.) I'd recommend unplugging it, and running for a few days.

Great thanks for the help Karma!

Nice idea, I'll go ahead and unplug it as soon as i get out of work, and tell you my results, I have to drive it for a few days as the car needs to stand for a few hours (cool off) to notice the problem.

Thank you!

Keep in mind that it will run pretty funky with the MAF unplugged, and throw a check engine light, so drive carefully and avoid WOT, but if your specific symptom clears up, you may have found the target.

Actually, I've never noticed it running weird without the MAF short of the engine light it will throw. On the 98 I was tracking down a crap wire from the MAF where it went through the firewall. So I ran with it unplugged for around 3 weeks. There was WOT in there as well, and it behaved very well. .. but every engine is different.

My '98 definately runs noticeably odd with MAF unplugged, especially when idling at low RPM. It acts like it wants to stall.

I've had a similar problem recently. Driving like a grandpa was fine but a sudden need of speed like entering the highway or trying to go uphill on the highway made the car feel odd. It was accelerate then draw back, accelerate drawback. I've replaced the TPS sensor but it remained. I finally broke down and bought the more expensive replacement MAF.

It worked! The 'power' is back. I can accelerate uphill without feeling a loss of power.

deekster_caddy wrote:

Keep in mind that it will run pretty funky with the MAF unplugged, and throw a check engine light, so drive carefully and avoid WOT, but if your specific symptom clears up, you may have found the target.

Ok well, yes, yesterday I did unplug the MAF sensor, then I started the engine and accelerate slightly to verify if that was the problem but immediately the CEL and the Traction Control off light came on, and well It did nothing different it just got worse the car was running like with more power I think because the extra fuel been injected. I guess if it would've been the MAF it will run poorly all the time and bad gas milage and I'm doing 19 MPG (mixed driving)

AA wrote:

My '98 definately runs noticeably odd with MAF unplugged, especially when idling at low RPM. It acts like it wants to stall.

and like Aaron said at idle the engine was running at about 300-400 RPM and then stall at the stop light it did that twice, so I didn't want to have any other problems with that and plugged the MAF back again. it only does it when I start the engine but as soon as the temp gauge goes a lil bit up (1-2 min after) it runs fine good acceleration and all.

I'm just curios, but does any body have the specs of the ECT? as far as ohms? because I checked the ECT and it does changes between temperatures, but I'm not sure if it is within specs

I would still try cleaning the MAF.

Other things to check -

Look hard for vacuum leaks - test spray carb cleaner all around the intake area and vacuum connections. Many of the rubber line-end connectors crack over time.
Fuel Pressure regulator - test fuel pressure on and off (while running with vacuum line removed)
Coolant Temp Sensor - read with a scanner and make sure it's reading correctly.

Update:

Today this noon it was about 66F, then I started the car and as always when it is in idle and I accelerate the engine it hesitates, then while doing so I shifted to OD and it acted pretty funny, the RPM went down to 400-500 and wanting to stall then shifted back to N and the RPM went way up to 1800 and then down again to 300-400 and then finally stalling... this was the first time it stalled , but this started to happen (hesitating more) when the outside temp are low like from 55F to 70F 2 days ago.
So yesterday I bought an electronics cleaner can and tomorrow I'll pull out the MAF and clean it, along with other checks like the TPS and CKP sensor, unfortunately I don't have a scanner and I will not be able to test the ECT appropriately.

deekster_caddy wrote:

Fuel Pressure regulator - test fuel pressure on and off (while running with vacuum line removed)

I'll try this too