Acceleration problem - Toyota Nation Forum : Toyota Car and Truck Forums

markdunn99 · 07-26-2011, 10:42 AM

Hi guys...95 XLE 6 cyl....when I step down hard on the gas the car bogs down.....as if it isn't getting any gas...and just sort of stays at the same speed....if I let up on the gas and just increase pressure slightly then it accelerates as expected. It's been sitting for a few months since I've been working on it....picked it up as a donor car to use as parts for my son's and now just getting this one back on the road....seems to run fine otherwise. I did have the MAF sensor disconnected after replacing the air filter...and had started it up that way...ran it for a while before noticing and reconnecting the MAF sensor...but the check engine light came on during that time and is still on now even though it is reconnected. Could that be messing it up somehow? Would disconnecting the battery reset the CE light - assuming there is no other "real" problem that it is sensing? Otherwise...I'm planning to clean out the throttle body.....iac valve if I can get it off....replace the fuel filter...what's the best way for the weekend mechanic to clean fuel injectors?? Thanks for any help....

hill8570 · 07-26-2011, 11:31 AM

Do the obvious first: if you've got a CEL, pull the code(s).

markdunn99 · 07-26-2011, 12:21 PM

Well sure....I was simply saying...the check engine light only came on after I had run the car with the MAF sensor disconnected....so it seems apparent that was the cause. It is now reconnected...so I wasn't sure if the CEL could be reset just by disconnecting the battery...and then I would go from there. If no more CEL then I know that was the cause .... Thanks

Do the obvious first: if you've got a CEL, pull the code(s

markdunn99 · 09-21-2011, 03:38 PM

[Just wondering if any other ideas on this? I'm back to working on the car...acceleration problem still there. If you stomp on it, it just sits and bogs down...but if you let up and just accelerate slowly then it seems to be fine. I've completely cleaned out the throttle body, removed and cleaned the intake manifold, new spark plugs, air filter....haven't replaced the fuel filter yet.....but was just hoping to get some other ideas. Is it a fuel pump problem? How would one verify if it's the fuel pump causing the problem? Thanks

QUOTE=markdunn99;3621103]Hi guys...95 XLE 6 cyl....when I step down hard on the gas the car bogs down.....as if it isn't getting any gas...and just sort of stays at the same speed....if I let up on the gas and just increase pressure slightly then it accelerates as expected. It's been sitting for a few months since I've been working on it....picked it up as a donor car to use as parts for my son's and now just getting this one back on the road....seems to run fine otherwise. I did have the MAF sensor disconnected after replacing the air filter...and had started it up that way...ran it for a while before noticing and reconnecting the MAF sensor...but the check engine light came on during that time and is still on now even though it is reconnected. Could that be messing it up somehow? Would disconnecting the battery reset the CE light - assuming there is no other "real" problem that it is sensing? Otherwise...I'm planning to clean out the throttle body.....iac valve if I can get it off....replace the fuel filter...what's the best way for the weekend mechanic to clean fuel injectors?? Thanks for any help....[/QUOTE]

Stillrunning · 09-21-2011, 04:11 PM

Did you clear the CEL?

markdunn99 · 09-21-2011, 06:46 PM

Yes....the CEL is cleared

Quote:

Originally Posted by Stillrunning

Did you clear the CEL?

Stillrunning · 09-22-2011, 05:06 AM

I'd check fuel pressure then TPS.

markdunn99 · 10-03-2011, 10:44 AM

I noticed I hadn't mentioned before that the ABS light is on....it was that way when I picked up the car. Put a new TPS in...but same problem...no change. Now I'm thinking it might be the VSS...speed sensor on the differential. Would that cause the ABS light to come on or would that only be related to the ABS sensors/system itself? I also didn't mention it is now exhibiting a shifting problem when driving.....sometimes it won't shift gears...have to release the gas then ramp back up slowly and then it will shift....that's why I'm thinking the VSS now. Not a cheap part to replace though. Guess I'll take it out and have a look...see if the manual shows a way for me to test it. Thanks for any input

Quote:

Originally Posted by Stillrunning

I'd check fuel pressure then TPS.

markdunn99 · 10-07-2011, 12:51 PM

Hi folks - any thoughts on this? I appreciate any feedback. Also want to mention...same problem exists whether in park or driving. Just sitting in park if I stomp on the gas sometimes it won't take.....it'll just stay at idle or slightly above...and then again....if I release the gas and then slowly depress it will ramp up accordingly and seem to be ok. Work done so far - replaced TPS sensor, completely dismantled and cleaned throttle body and intake plenum, new spark plugs, cleaned iac valve, new air filter, new fuel filter. Any help is appreciated. Thanks

Quote:

Originally Posted by markdunn99

I noticed I hadn't mentioned before that the ABS light is on....it was that way when I picked up the car. Put a new TPS in...but same problem...no change. Now I'm thinking it might be the VSS...speed sensor on the differential. Would that cause the ABS light to come on or would that only be related to the ABS sensors/system itself? I also didn't mention it is now exhibiting a shifting problem when driving.....sometimes it won't shift gears...have to release the gas then ramp back up slowly and then it will shift....that's why I'm thinking the VSS now. Not a cheap part to replace though. Guess I'll take it out and have a look...see if the manual shows a way for me to test it. Thanks for any input

Stillrunning · 10-07-2011, 02:21 PM

Quote:

Originally Posted by Stillrunning

I'd check fuel pressure then TPS.

Checked fuel pressure yet?

Quote:

replaced TPS sensor

Why didn't you check it first?

Quote:

Originally Posted by markdunn99

I noticed I hadn't mentioned before that the ABS light is on....it was that way when I picked up the car. Now I'm thinking it might be the VSS...speed sensor on the differential. Would that cause the ABS light to come on or would that only be related to the ABS sensors/system itself? Not a cheap part to replace though. Guess I'll take it out and have a look...see if the manual shows a way for me to test it.

You can get trouble codes for ABS system - see manual. The VSS sends signals after/during speed changes, it does not determine fuel input for acceleration as then there would be a lag. No where in my ABS troubleshooting does it mention VSS.

Quote:

Originally Posted by markdunn99

Also want to mention...same problem exists whether in park or driving. Just sitting in park if I stomp on the gas sometimes it won't take.....it'll just stay at idle or slightly above...and then again....if I release the gas and then slowly depress it will ramp up accordingly and seem to be ok. I also didn't mention it is now exhibiting a shifting problem when driving.....sometimes it won't shift gears...have to release the gas then ramp back up slowly and then it will shift....that's why I'm thinking the VSS now.

If you put the gear selector in 1st then accelerate quickly and shift manually, do you get the same non-response? Happening in park does not lead me to VSS or trans shifting problem. Does it almost stall when first stomped on or does the rpm just stay at idle?

markdunn99 · 10-07-2011, 02:56 PM

Thanks for the response. No haven't checked fuel pressure yet. Not sure I have the means to do so? Guess I could pick up a fuel pressure check kit right? And....what would it mean if fuel pressure is not correct? Regulator? Fuel pump? Didn't check TPS first only cuz it's simple to replace and relatively inexpensive...I don't mind throwing a couple of new parts in an older car if it's not going to break the bank. If it doesn't fix the problem...at least you know you have a new part in there that shouldn't give you any trouble in the future. I will try driving it and shifting manually to see how it responds then...I haven't tried that yet. It seems to want to try and stall when stomped on...but it never has actually stalled....it just sort of bogs down and then stays there with no additional ramp-up unless I release the gas and then depress slowly.

Quote:

Originally Posted by Stillrunning

Checked fuel pressure yet?

Why didn't you check it first?

You can get trouble codes for ABS system - see manual. The VSS sends signals after/during speed changes, it does not determine fuel input for acceleration as then there would be a lag. No where in my ABS troubleshooting does it mention VSS.

If you put the gear selector in 1st then accelerate quickly and shift manually, do you get the same non-response? Happening in park does not lead me to VSS or trans shifting problem. Does it almost stall when first stomped on or does the rpm just stay at idle?

Stillrunning · 10-07-2011, 05:20 PM

Quote:

Originally Posted by markdunn99

It seems to want to try and stall when stomped on...but it never has actually stalled....it just sort of bogs down and then stays there with no additional ramp-up unless I release the gas and then depress slowly.

To me that indicates not enough fuel while there is more air since throttle plate is open. When the throttle is opened quickly there needs to be a shot of fuel to respond to the extra air passing the throttle plate, that's what the TPS is for, to tell the computer to go rich for a second and prevent lean bogging - my understanding anyway. On carbureted vehicles that was accomplished with the accelerator pump, the theory is no different with fuel injection.

Maybe your TPS connections are not good?? Try the manual shifting and see if trans related. Advance Auto or Auto Zone (etc.) should be able to loan fuel pressure gauge.

***fenixus*** · 10-07-2011, 08:00 PM

TPS is one thing, but on electronically fuel injected engines, it's the computer that commands wider fuel injector pulse width when needed. it detects intake air volume/pressure using MAF and/or MAP sensors.

but it also takes into account upstream exhaust oxygen sensor reading, if it doesn't see tons of oxygen through it then it won't attempt increasing the injector pulse width (since AFR looks good from its point of view)... so ... how old are the upstream oxygen/AFR sensors?

they could be lazy/slow and could easily be responsible for the initial acceleration lag and bog down and then behave quite normally for a while.

other than that, if the RPMs do not really rise, I would check if the accelerator cable is not perhaps binding somewhere. have seen something similar on an older V8 Ford (or GM was it?) truck at old work place. you had to literally briefly stomp on gas pedal from time to time, because it wouldn't respond at all initially when pressing it gently.

Stillrunning · 10-07-2011, 08:28 PM

Quote:

Originally Posted by fenixus

TPS is one thing, but on electronically fuel injected engines, it's the computer that commands wider fuel injector pulse width when needed. it detects intake air volume/pressure using MAF and/or MAP sensors.

I was looking at required response time, but don't know how fast MAF to computer signal could react. By the time air flow went past MAF it would be too late for computer to send more fuel signal?? TPS would send instant output to ECU to increase fuel for airflow. We (at least I) do need computer nerd here to set us straight on input/output and reaction times. Why is there a TPS other than for idle/off idle output to ECU?

***fenixus*** · 10-07-2011, 08:43 PM

^ no idea, really. but I think we are talking about mili- or micro-seconds here.

it's probable that MAF output reaches the ECU tiny bit later (air needs to pass through the sensor wire) than the TPS output (instant once you move the accelerator pedal) ... still the difference could be irrelevant ... and the increased air flow (detected by MAF) is the reaction to engine vacuum thanks to wider opened throttle plate (which happens in first place). but does it really matter for the ECU? I don't know.

... or maybe the MAF is dirty and doesn't report the proper air volume which messes up the ECU work? who knows. things need to be cleaned/checked.

I would clean the MAF, it's very easy, you don't even have to remove it, just unbolt and unplug the top cover of the air box and flip it upside down, you will see MAF through intake opening.

and at least measure the resistance of upstream oxygen/AFR sensors to see if they are anywhere near the specs at 20C/68F (if you can make them fall in or close to that temperature range). but if they are older than 150k miles I would just replace them with new ones regardless.