I've had my truck about 5 weeks now and it's great. But, it has a bit of a hesitation or stumble, as I call it, when I come off idle to accelerate from a stop. I just give it a little throttle to start out and it dies down a little then picks up and goes like normal. It's more pronounced on a cold engine but will do it most times even warm. Almost like in the carb days when an out of tune car would flood a little then take off after it burns off the fuel. I just put in new plugs, wires, rotor, and cap. This gave me about a 1 mpg gain. Since my tune up I've gotten 16.6, 16.6, 16.4, 17.5, and a 13.3 but I was very heavy in the throttle. It runs great but this stumble seems like something I should get rid of.

Thanks.

 

sounds like a TPS to me . or the IAC valve needs cleaned?? possibly even a vacuum leak?

 

Ya, climb in there and check all the hoses you can get at. How are your plugs and wires? The tps can be checked for flat or open voltage spots with a digital volt meter, I"m not sure what is power and which is signal wire. A Haynes Manual might help you trouble shoot also.

 

check voltage on the TPS at idle position with the green and black wires. then check it at WOT . it should be right around 1 volt dc at idle or slightly under and up to 4.0 + volts at WOT. chiltons calls for .66vdc to 1.22vdc idle and 4.86 max @WOT. orange wire is supply voltage and should be right around 5 vdc.

 

The best method for determining if the TPS has the kind of flat spot that would cause the conditions you described is with a DMM with a bargraph. Check the idle and WOT settings, and post them here, along with whether or not you have a DMM w/ a bargraph.

Take care,
-Chris

 

Unfortunately I do not have a DMM. What is a DMM? I'm pretty handy with cars but I don't mess with car computers much. Maybe I should pick up some equipment and learn. Would the computer code reader tell me anything? I don't have one but might need to get one of these too. What brand code reader would you recommend?

I have all new plugs, wires, cap, and rotor. FYI, the truck had the stumble before my tuneup so I don't think the ignition parts had anything to do with it. I routed the wires carefully but may doublecheck them.

Thanks.

 

Dmm, digital multimeter.
Lets think for a minute...
94 351...
Hey HO guys, didn't the HOs have an issue with close wires too? Like 4&5 or 5&6 or 6&7?

Maybe timing is to far back. I'm stumped, I need to be there.

 

The wires to check on a 351 are 5 & 6.

Code reader? don't spend less than $30, or more than $200, and you'll be fine.

 

Thanks for the info. Which ones are 5 & 6? Actually, I can find the #1 cyl by following the wire from the cap but how are they numbered after that? I mean, which ones are 2-8? I'll have to write this down for future reference.

If I had a DMM what part would I be testing to diagnose the stumble? The TPS? Is this easy enough for a novice to do? I need an MM anyway for around the house so this might be a good excuse to get on.

I just ordered the Chilton's manual and the reviews on Amazon were pretty bad. Will this book help me with the various sensor tests so I can do them myself or is the book as bad as the reviews said it is?

 

The cylinders in Ford V8's are numbered 1-4 and 5-8, front to back, starting on the passenger side. The two wires that you MUST have completely separated, from the distributor cap to the plugs, are numbers 5 & 6. They are the forward most cylinders on the drivers side.

A DMM is a must. I would HIGHLY recommend buying one that also has a bargraph. That means that it will have a series of dots/lines that run from one side of the display to the other, and it will act as a visual monitor when you are measuring a circuit with a changing value. For example, you've seen digital speedometers, right? They can be tough to read if your speed is changing rapidly, so most also come with a bargraph so that you can see a line move to represent the speed. This is imparitive for good diagnostics. The two problems that occur with TPS', are that it shorts, and has full voltage (or no voltage), or, more commonly, it wears out. It's nothing more than a potentiometer, which is just like the dimmer switch in your parents dining room. The TPS turns alot more than your parents switch, though! Anyway, areas that see the most use are prone to wearing the contact surface , and you get a jumpy throttle. This is usually tested for by applying voltage to the TPS, and monitoring it's change while rotating the throttle. problem is, voltage is a terrible way to measure resistance. resistance should be measured with ohms. If you get a DMM with a bargraph, do this:

Disconnect the TPS at the harness, and attach the leads of your DMM to the pins that plugged into the green and black wires. Set your DMM to ohms, on the 10 scale, and very slowly rotate the throttle. Watch the bargraph while doing this. It should never backtrack. It should only go forward, as you open the throttle. If the TPS is worn, the bargraph will get "jumpy" when you reach the worn area. If this happens, replace the TPS.

Chiltons is good for general maintenance. It will tell you what your sensors are, and will probably give you one or two tests for them, but not very good tests. You should go to barnes and Noble, and look for a book that specifically deals with Ford DTC's (Diagnostic Trouble Codes).

Take care,
-Chris

 

I thought that this was the wiring scheme but wanted to double check. I took some time tonight and made sure ALL my wires were well separated. I took some of that plastic sleeving, the kind that comes on the stock coil wire, and covered all wires from the distributor to the first set of plastic separator clips. This should insure that no contact between wires can take place. I hope this was a good idea. The factory coil wire came with this stuff on it but my replacement wire did not so I bought some and went ahead and put in on all the wires.

If I pick up a DMM I'll let you know. If it persists I'll have to do something.