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Discussion Starter #1
I've got an 88 that will start right up and idle fine, but stumbles as soon as it accelerates. It will rev up fine in park. FPR has been replaced (same symptoms a few months ago). Fuel pressure is 40psi key off, 31psi running. Rev it up and it will go up to 34psi and fall back down again. If I pull the vacuum line to the FPR it goes back up to 40 psi. I replaced the FPR again, and get the same results. I've seen several references to fuel pressure on this site, but I haven't seen a solid set of numbers for what I should be looking for.

No vacuum leaks that I could find. Soaked all the lines with carb cleaner and didn't see a difference in idle. Also, KOEO tests shows everything passed.

At this point I'm at a loss. This is my DD, and I've got limited time and budget to get it running right.
 

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Discussion Starter #3
I thought about the TPS, but it revs up fine when in park. Seems a TPS issue would stumble in park as well, or toss a code...
 

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Does it stumble only when under load, or whenever you are moving? Would it stumble if you press the accelerator while going down hill (not under load)?

Im just wondering if it's not an engine issue. You said there are no codes... maybe its something else in the drivetrain?
 

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I'd start by cleaning the IAC = idle air control valve located on the right side of the throttle body using throttle body cleaner which evaporates nicley. Remove the 2 bolts...8mm ?, unplug the harness connector and it comes right off, check the gasket and replace if need, in some cases the electric solenoid can be seperated from the valve, 2 small screws and only clean the valve/bottom part assuming you dont take the solenoid off otherwsie you could fry it.

Back probe the TPS and see what's up there and check to see if you have the TV.Cable for the transmission, it's a very simple adjustment, You Tube = TV cable adjustment video.


There is a Ford Racing Performance Parts "idle setting procedure" Instruction Sheet (Techline (800) FORD788) steps 1-9 on How To adjust idle by disconnecting the IAC using the throttle body idle stop screw, drilling out the mounting screw holes on a newer "plug & play" TPS so you can adjust it for voltage setting.

In the past the ideal setting was thought to be .93-.97 volts or just under 1 volt but recently posted information regarding this appears the setting should be between .55v-.75 volts.

Posted by FSB member Jermil01 recently:

Wanted to update this thread based on some TPS adjustment information I got from one of my tuning sites. This goes against the conventional wisdom of setting the TPS closer to .95 range. Thoughts??

Regarding any ECU controlled function (and idling conditions in particular), before any strategy based adjustment can be made by a computer controlled system, it must first run somewhat normally in a default and OL mode.

To do this with a FORD you must first follow a few rules.

You must keep the TPS input above the deadband area and below the "tip in" area. On a FORD this equates to below approximately 0.9V and above 0.45. Anything above 0.90V the ECU thinks the throttle is starting to open. This brings into affect possible dashpot and fuel cut strategies which can turn one grey real fast. Below approx 0.40-0.45V the system suspects an incorrect input and substitutes a default figure.

So first off, the TPS should be set to around 0.55-0.75V. You can also disconnect the IAC then disconnect the battery for 15-20 minutes, reconnect and the computer should default back to factory settings.


Good Luck ~ :thumbup
 

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Discussion Starter #6
I'll pull the IAC and clean it up. Since it idles fine, I didn't think the IAC would factor in. I'll also check the TPS.
I've got the C6 - I didn't think they had the TV cable
 

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If it's a C-6, they don't. Wonder how your ignition components are. I would start with examining the plugs, wires, cap and rotor.
 

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Discussion Starter #9
Cleaned IAC, backprobed TPS, checked MAP sensor, cleaned rotor, replaced cap. All plug wires look OK, and have less that than 5k on them (Taylor 8mm wires).

Still no codes. It does seem to run better after the IAC, and definitely idles smoother.

It will rev to red line in park without a problem. Stumbles/misses when accelerating.
 

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Wonder if the cat is plugged, or your fuel filters are plugged. your model year should still have 2 filters on it.
 

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fuel filters?
 

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I'll swap the fuel filters tomorrow when the rain lets up. I'm running out of ideas...
+2 on fuel filter(s) - had this exact problem with my boat this year. Have you checked fuel pressure as well? I might have missed that in the above so if I did, please forgive!;) Ignore that, re-read original post. If the filters don't fix it, might be worthwhile to just double check the flow into a bucket or something. Good to rule it out completely.
 

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I would also check for codes with the engine running.
 

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Is the coil new or has it ever been changed? Stumbling under the load makes me still think something on the fire side of things.
 

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Discussion Starter #18
Swapped fuel filters and did a full tune up with all MSD parts (coil, wires, plugs, cap/rotor). Idles and revs like a champ, but still stumbles between ~1200 and 2000 RPM when driving. After 2k it runs fine unless I really floor it. Manually shifted in 1 and 2 and its the same thing.
 

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Your TPS is a rotory potentiometer that changes voltage in relation to the Throttle Plate Shaft. This is a part that does wear out, usually by creating a dead spot in the most used lower range. Typically a TPS dead spot will not set a DTC. This dead spot can create a hesitation or stumble like what you are experiencing. I know you stated that this does not stumble in Park, but this would be more prevalent under load. Note: Do not attempt to manually set your TPS or adjust your Throttle Plate as referenced above as this is incorrect information.

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To test the TPS, you must use a DVOM I recommend using an analogue meter as you can immediately see voltage changes that would indicate a dead spot where a digital meter will have a delay and be more difficult to see.



First unplug the harness connector and with key-on, use a DVOM to test the VREF and SIG RTN terminals for 5v Reference voltage.

Next, plug the TPS harness connector back in and, with key-on, back-probe the TP SIG and SIG RTN terminals to ensure you are between 0.73v and 1.22v. Anywhere should do as your TPS is self-calibrating and non-adjustable. Then, watching your meter, slowly rotate the Throttle Plate and look for a smooth increase in voltage to approximately 4.5v.

Finally, turn the key off and once again unplug the harness connector. You will do the same Throttle Plate rotational test again back-probing the TP SIG and SIG RTN terminals, except this time testing resistance. You should start at just less than 4K ohms and then decrease to just above 350 ohms.

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If you find that any of these voltage or resistance transition are not smooth, replace the TPS and disconnect your battery with headlights on to erase any adaptive learning strategies. After reconnecting your battery, you will have to idle for for a period of time and then drive at cruising speeds for your PCM to relearn. Also, be sure to look in your Throttle Body bore for signs of contaminant build-up that would make the Throttle Plate stick.

For more information, see this link: Throttle Position Sensor Management


 

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Discussion Starter #20
TPS has been swapped out. It idles smoother, but it is still acting up. Now it will stumble in neutral too. I've got the feeling the fuel pump is dieing. It feels like its running out of gas. I don't mind swapping it out, hopefully I can get to the bottom of the wonky fuel gauge while I'm at it. It doesn't hold pressure as you rev it up. It'll go up about 3 psi, then fall back down.
 
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