92 Ford F150 bogs down/loses power

[warmachine5500]

This isn't really urgent, but I was unsure where to put an F150 question in a Ranger forum.

92 Ford F150 5.0 EFI motor. My friends moms truck needed a new throttle position sensor (it was idling high). Her grandson put it on. Now it idles fine, but when you want to put your foot in it (going up a hill or getting on a highway) the truck will bog down. It loses power, a check engine light appears, and if you keep your foot in it it will eventually stall while you're driving. If you take your foot off and let it "breathe" the engine will idle like its supposed to, but if you try to hammer down again, the same thing will happen. It only happens when you put your foot in it. If you're very gentle on the throttle, it won't happen.

I looked around the TPS and couldn't find anything wrong or disconnected. I covered the throttle body intake with my hand and it stalled (checking for a vaccuum leak).

Is it possible the new TPS sensor is doing this? I would think if it were bad out of the box, it would idle funny. I have a feeling it's not related. Possibly a fuel pressure issue? Weak fuel pump? I took a look at the fuel filter. It looks fairly new. I have to get my hands on a fuel pressure tester I suppose, but I'm open to any ideas! Thanks!

 

[RonD]

Low fuel pressure first shows up at highway speeds or sustain load, like going up a longer hill, extra fuel demand.
Just accelerating bogging would be when pressure was way way down, you couldn't drive it at highway speeds.

TPS is the same as the accelerator pump on a carb.
Accelerator pump gave the engine an extra squirt of gas when you pressed down on the gas pedal, this allowed the main Jets to "catch up" with the increased air flow and RPMs.

So your thinking is correct, TPS should give fuel injection computer a "heads up" to add extra fuel until MAF sensor has a chance to "catch up" to increased air flow.

But like the accelerator pump the TPS doesn't set air/fuel mix, or even used for air/fuel mix, it is just a performance enhancer.
TPS does have other uses but they don't apply to these symptoms.
TPS is a "learned" sensor, so does need to be in place for a few drive cycles, heat up/cool down = 1 drive cycle

Putting your hand over intake can test vacuum level, but on fuel injection a better way is to use the IAC Valve since it controls air stream past throttle plate.
After engine is warmed up unplug IAC Valve, it should close and idle should drop down to 500rpm or engine may stall, either is good it means IAC Valve is working and there are no vacuum leaks.
If idle stays at 700 or higher then either there is a vacuum leak or someone has been messing around with the TPS adjuster/anti-diesel screw.
This "looks" like an idle screw but is NOT.
Fuel injection can use an idle screw, no jets to add extra fuel with air flow.

MAF sensor could be dirty.
It is the main air/fuel mix sensor.
When you open the throttle the TPS gives the computer the "heads up" and computer will add extra fuel for a second or two, but MAF sensors air flow must also show an increase or engine will just bog down.

Unplug the MAF sensor and see if engine seems to run better, it will be sluggish but not like you describe.
Without MAF sensor hooked up computer will use RPM as fuel mix reference.
CEL will also come on, which is good it means computer is monitoring air flow.

 

[warmachine5500]

I don't think this engine has a MAF sensor. Shouldn't the MAF be inline with the main intake hoses going from the air filter box to the throttle body? There's not one there. However I think there's a MAP sensor, located on the firewall.

The only thing I see that looks like an idle adjustment screw is in this image, located on top of the throttle body. Turning the screw in would hold out the throttle plate, allowing for higher idle. Just like stepping on the gas pedal/pulling the throttle plates open. Is this the screw you're referring to? :

https://www.google.com/imgres?imgur...szg893MAhWDMj4KHQCpCRkQxiAIAygB&iact=c&ictx=1

I didn't take the truck on the highway. I got it up to about 60, but I doubt it would be able to sustain highway speeds, especially going up hills. I'll try your IAC trick, but I don't think there's a vacuum leak. I can't get the engine to bog at any RPM in park or neutral. It's only when it's under load. I think she's been driving the truck like this for a week or two, so that should be plenty of time for the new TPS to learn itself. I suppose I could try unplugging the new TPS and see how it runs without it?

 

[RonD]

Can't quite see it but yes, it is adjusts throttle plate position thats the anti-diesel screw, it can also be used to adjust TPS closed throttle voltage, .69-.99volt

MAP sensor was used on earlier Speed Density systems, I guess '92 is "earlier", lol.

MAP sensor can be tested which would be the next step, assuming vacuum hose for MAP has been checked.
MAP sensor reads air pressure when key is turned on, i.e. elevation, after start up it reads vacuum level in intake, high vacuum means less fuel is needed, lower vacuum more fuel, engine has lower vacuum under load.

Speed Density runs air/fuel from a table, RPM/Vacuum, then after warm up O2 sensor data modifies it a bit.

So MAP sensor is important to establish correct air/fuel mix.

TFI ignition I would assume, bogging could also be a spark advance issue.

 

[warmachine5500]

Is that anti diesel screw the only way to adjust the TPS? Or is it also adjustable on installation? I wonder if her grandson didn't pay attention to the way it came off, and just put the new one on any old way.

 

[enjr44]

I'm back to how a TPS can cause a high idle. It is just a variable resistor that tells the computer where the throttle is positioned to adjust injector pulse width. Idle speed is controlled by the IAC, it is sort of like (but not exactly) the choke on a carbed engine. All open about 1500 rpm. Completely closed less than 500 rpm. The TB butterfly position could raise the idle speed if adjusted improperly. How did the GS decide the TPS was bad?? The only way I know to test it is with a volt meter.

Like Ron said, pull the connector off the IAC and see what happens.

 

[warmachine5500]

From what I was told, someone else tested the original TPS and told her it was bad. The grandson simply changed it. Oh how I love cleaning up others messes lol.

I had a few minutes to look at the truck last night. I simply unplugged the TPS. Of course it's throwing a code now, but I couldn't get it to bog down like it was doing with the TPS connected. I didn't have much time to drive it. The local cop started eyeballing me, probably because I was cruising around the 25mph main street getting it up to 50mph so I left it. Today the owner told me she drove it and she couldn't get it to bog down either. I'm thinking about putting on a different TPS and seeing where that gets me.

 

[Tedybear]

From what I was told, someone else tested the original TPS and told her it was bad. The grandson simply changed it. Oh how I love cleaning up others messes lol.

I had a few minutes to look at the truck last night. I simply unplugged the TPS. Of course it's throwing a code now, but I couldn't get it to bog down like it was doing with the TPS connected. I didn't have much time to drive it. The local cop started eyeballing me, probably because I was cruising around the 25mph main street getting it up to 50mph so I left it. Today the owner told me she drove it and she couldn't get it to bog down either. I'm thinking about putting on a different TPS and seeing where that gets me.

I'd verify it works or not first. I was sold a defective one a while ago so it can happen. The return spring was broken and it would stick where ever the throttle was last opened at..... Simple enough to check with a multi meter and install correctly. (of course, verify the last guy that worked on it changed the part with the correct one...)

S-

 

[RonD]

Unplugging a sensor like the TPS can put computer into OPEN LOOP this means it will run Factory air/fuel mixed and not rely on any feedback from O2 sensors, so it isn't a sure sign a sensor is bad if engine runs better when it is unplugged.

Testing sensor with volt meter is the best way.
tells you if computer is sending correct voltage out, 5 volts
and if TSP is sending back correct voltage, .69 to 4.8volts

 

[warmachine5500]

So the little plastic gear inside the sensor should be spring loaded? That sounds like something to look into.

 

[warmachine5500]

Tedybear I owe you a cookie.

I went to Grandma's house, took the throttle body off and removed the sensor. Lo and behold, no spring action.

It was a Valuecraft brand (Autozone, Cee Kay), so I went to Advanced Auto Parts, got a new Carquest brand sensor and throttle body gasket. I installed them and voila like frickin magic it had plenty of power (for a 24 year old neglected 5.0), no bogging down, and no check engine lights. I imagine the sensor was bad out of the box. I don't think the grandson could have broken that while installing it.

Thanks everybody for your help and input! Grandma is very happy!

 

[Tedybear]

Glad it was a simple fix. Annoying that a new part is defective, but simple.

On our old BII the TPS I was sold appeared to have been cleaned up and reboxed. When I talked with the mgr of the parts house he mentioned it happens a lot. Someone will install the new part, clean the snot out of the old part so it looks 'new' and return it stating "we didn't need it". So it goes back onto the shelf.

S-