Acceleration issue?

[Brian1973]

Does it make any sense that when I push my foot all the way down I get acceleration that feels normal but if I let off the pedal just an inch or so I feel the engine accelerates slightly more? What would cause that?

 

[Brian1973]

Thinking about it, not sure if this says anything, but when I use cruise control on this truck, I can always push down an extra inch or so when going up hills while cruise is engaged... sometimes it seems to help... which sorta conflicts my previous question... ????

 

[swynx]

My truck did this before I replaced 02 sensors distributor and wires. It was very weird.

 

[RonD]

Well a couple of things come to mind.

With throttle plate open all the way(WOT) you might be getting too much air for fuel air mix(lean), letting off the pedal a bit gives better fuel air mix.
Now that being said a lean mix should cause pinging under load.
So maybe there is something else at work.
When computer gets the WOT voltage from TPS(throttle position sensor) it goes into Open Loop, so disregards O2 sensors and runs a preset fuel/air mix based on MAF airflow and temp readings, or MAP vacuum and air temp readings, so could be a MAF/MAP issue.

WOT relay also closes so disables A/C compressor for more power to wheels.

I don't think cruise control can pull throttle to WOT position, that must be done by driver.

 

[Brian1973]

Very interesting RonD...

I'm thinking too that I notice it more around the time I have another issue... Exhaust Manifold leak that burns through a spark plug wire... (http://www.therangerstation.com/forums/showthread.php?t=146901)

It's hard to pinpoint and so faint really that most times I don't take notice to it and it's hard to tell because of road incline or other variables... I get NO pinging...

When I burn through a plug wire I definitely feel the issue because I get the "miss" at that point where I release the gas that inch... (btw, this is highway driving... not around town that I notice). The miss will go away "usually" when I smash the pedal... but then will get hardly any acceleration.... sorta like it's just firing the 5 cylinders and not sure where the 6th's fire is going... let up an inch, and I feel the "miss", spark grounding to the engine... but when the spark goes to the plug I feel better acceleration than smashing the pedal... I would point it exclusively to this spark culprit but it seems to be generally all the time...

Would a MAP (manifold absolute pressure) issue be affected by an exhaust leak? (I think it studies the intake manifold doesn't it?) I've had the issue so much in the last few years yet I fix the plug/wire issue with another wire shortly after noticing the miss... and yet still seem to notice this variable in the positioning of the pedal.... I'm not up on my emissions terminology or what all I have, 1991 have one or the other or both? MAF/MAP... and would it be something to just change out at 170k to cancel the possibility? My TPS I do recall cleaning out in the past because of some issue that I can't recall offhand... maybe rough idle? when I hadn't figured out the burned spark plug issue...

And the biggest thing I got out of what you said was the WOT relay disables the A/C compressor... 100% positive on that? I've never had it do that I recall lately.... if compressor is engaged, the truck is a COMPLETE dawg... compared to just being a bit of a dawg normally... I definitely turn the A/C off if I'm going up a long hill on the highway and need some steam! Does that point to some issue then if compressor doesn't shut off at WOT?

 

[swynx]

Interesting that ron said that because I also replaced my iac and Tps at the same time

 

[RonD]

Well the Ranger throttle cable can stretch over time, so maybe you are never getting WOT, so relay never shuts of AC.
Engine off hood open, remove throttle cable cover, have someone push the gas pedal all the way down, then see if you can manually open throttle plate more.
If so you need the Ranger throttle cable "mod", in essence you lift up on the gas pedal to expose the end of the cable above it, then put a wire tie(zap strap) around the bare part of the cable, this in effect shortens the cable, so you have WOT back.

I think you cleaned the IAC valve, Idle Air Control Valve, they do need cleaning and effect the idle.
TPS, throttle position sensor are usually not cleaned, connectors can be, but not usually not the sensor itself.

 

[Brian1973]

or maybe the relay isn't working right? I will check the throttle cable as you described

 

[RonD]

Could be, it is a mechanical relay.

WOT relay connects A/C cycling switch to A/C compressor and is normally passing that power when A/C is on, so relay is N/C(normally closed) if not activated

Relay has 12v power when key is on(from EEC relay), but no ground so it stays N/C
Computer(PCM) grounds the relay when TPS shows WOT voltage(approx. 4.9v), this "opens" the relay breaking contact, so power to A/C compressor is cut while this relay is grounded by PCM.

 

[Brian1973]

throttle cable not stretched... does open to WOT when pedal fully pressed.... compressor definitely stays ON at WOT... guess I will check into this other issue...

the new wires seem to rectify the slight acceleration felt when the pedal is lifted slightly.... must be that while the engine doesn't appear to be "missing", the wearing away of the boot from escaped exhaust gasses must cause some of the current to not fully reach the plug at WOT.... therefore, releasing the pedal slightly causes the complete spark to go where it's suppose to? making it appear more acceleration when foot is lifted...