Part Throttle Miss

[seery]

Hey Guys,

I've finally had the chance to play with my 88 Shelby Z lately, been a few years since it's been out of storage other than the occasional start up. It's acting a little funny though..

During normal driving, basically just starting to push the throttle maybe 1/4 of the way, it likes to miss a little.. after that it's fine. It's a little annoying when pulling out and in between shifts. Other than that, it runs smooth and pulls hard. I had it out all weekend and it didn't improve at all after running it around a good bit.

Anyone experienced this and know what it may be? I'm thinking a sensor, but it's not throwing any codes.

Thanks!

[NAJ]

Do not jump into Fuel Management/Computerized Engine Control systems until you have thoroughly checked all of the Basics.
How Old is the Fuel? What is the Condition of Secondary Ignition Components?
Are the Fuel Injectors Dirty?

[seery]

Sheesh.. I miss the old days on the shelby dodge mailing lists when people were more pleasant... this reminds me of honda-tech.

If this should be in another section, could it be moved?

Anyways, thanks for the reply. I did check basics..

The fuel is new, the plugs look good, cap/rotor and wires are basically new as well. Checked vac lines, everything looks okay - nothing dry rotted or cracked.

You mentioned the injectors. I'll try pulling the fuel rail and checking them out, then running some injector cleaner through.

Thanks for the tips.

Quote:

Originally Posted by NAJ Do not jump into Fuel Management/Computerized Engine Control systems until you have thoroughly checked all of the Basics. How Old is the Fuel? What is the Condition of Secondary Ignition Components? Are the Fuel Injectors Dirty?

[NAJ]

I do not think you need to pull the fuel rail,if there is a dirty injector you will only find it by running an Injector Balance Test. If this is the problem Aftermarket in the Tank cleaners may not be strong enough.You may have to have the Injectors cleaned with cleaner that goes directly into the Fuel Rail.

I do not understand the top portion of your Post,You thought I was being unpleasant? I am only trying to help you diagnoise and correct your problem without throwing parts at it.

[seery]

I guess I just didn't understand why I was being told not to post in this section before any discussion was started about the problem.. felt like I was barked at for doing something wrong. The help is very much appreciated though.

To do an Injector Balance Test, isn't that pulling injector connections, one at a time, at idle then noting differences? (at least according to a post found I found from a search.) I'll give that a shot to see what I find. It does seem ok during idle but it's when you first hit the throttle that it wants to hiccup - after that it smooths out.

Thanks again.

Quote:

Originally Posted by NAJ I do not think you need to pull the fuel rail,if there is a dirty injector you will only find it by running an Injector Balance Test. If this is the problem Aftermarket in the Tank cleaners may not be strong enough.You may have to have the Injectors cleaned with cleaner that goes directly into the Fuel Rail. I do not understand the top portion of your Post,You thought I was being unpleasant? I am only trying to help you diagnoise and correct your problem without throwing parts at it.

[NAJ]

You are refering to a cylinder balance test to determine which cylinder may be at fault for a rough running/misfire condition but you would still have to perform diagnostics to determine the cause.(mechanical,fuel,ignition)
An injector balance test requires the use of a fuel pressure tester and injector tester. It will determine if all injectors are delivering the same amount of fuel. You install the fuel pressure gauge,cycle the key to get the gauge to max regulated pressure, the injector tester plugs onto the battery and onto the injector,you then activate the tester which will pulse the injector and you watch the drop in fuel pressure,repeating the process with all injectors.
(you have to be sure fuel pressure is reading the same in all cylinders or your results will not be accurate)
If there is a dirty injector fuel pressure will not drop as much in that cylinder.
They lowest should be within 5% of the highest.The controller has no idea how much fuel is actually being delivered,it assumes that if it keeps the injector open for a certain amount of time (pulse width) a certain amount of fuel will be delivered.
I work at a Jeep Dealership and dirty injectors are still responsible for 50% of cylinder misfire codes on OBD2 vehicles. Even with the detergents in todays gasolines it is still a carboned based fuel and the injector is still exposed to extreme engine heat,put that together with a pintle style injector and you have problems.
I have an OTC injector tester that I have had since the mid 80's and it allows you to pulse the injector with 1 large pulse,50 short pulses or 100 shorter pulses in a preset(in the tester program) amount of time.
I am not saying that this is your problem but a part throttle misfire may be difficult to track down without the use of an O'Scope to see each individual cylinders readings when the problem occurs.There is going to be trial and error and that is why I suggested checking all basics first.

[seery]

OK, I got ya.

I think I can track down injector tester to borrow and I should be able to put a fuel pressure gauge on the rail. Thanks for the tips, definitely worth a shot.

I haven't been able to get down there to try anything yet, hopefully this weekend. I've also been considering any other possibilities... would a failing throttle position sensor cause these symptoms? I'd expect a code for that though. The stumple when you first tap the gas is just more annoying than anything. It's not like I'm worried about track times, etc. But I do want to get it taken care of.

Thanks NAJ.

--M

Quote:

Originally Posted by NAJ You are refering to a cylinder balance test to determine which cylinder may be at fault for a rough running/misfire condition but you would still have to perform diagnostics to determine the cause.(mechanical,fuel,ignition) An injector balance test requires the use of a fuel pressure tester and injector tester. It will determine if all injectors are delivering the same amount of fuel. You install the fuel pressure gauge,cycle the key to get the gauge to max regulated pressure, the injector tester plugs onto the battery and onto the injector,you then activate the tester which will pulse the injector and you watch the drop in fuel pressure,repeating the process with all injectors. (you have to be sure fuel pressure is reading the same in all cylinders or your results will not be accurate) If there is a dirty injector fuel pressure will not drop as much in that cylinder. They lowest should be within 5% of the highest.The controller has no idea how much fuel is actually being delivered,it assumes that if it keeps the injector open for a certain amount of time (pulse width) a certain amount of fuel will be delivered. I work at a Jeep Dealership and dirty injectors are still responsible for 50% of cylinder misfire codes on OBD2 vehicles. Even with the detergents in todays gasolines it is still a carboned based fuel and the injector is still exposed to extreme engine heat,put that together with a pintle style injector and you have problems. I have an OTC injector tester that I have had since the mid 80's and it allows you to pulse the injector with 1 large pulse,50 short pulses or 100 shorter pulses in a preset(in the tester program) amount of time. I am not saying that this is your problem but a part throttle misfire may be difficult to track down without the use of an O'Scope to see each individual cylinders readings when the problem occurs.There is going to be trial and error and that is why I suggested checking all basics first.

[NAJ]

Yes it is possible that a TPS could cause this problem if there is a dead spot.
You can use a scanner or digital voltmeter connected between the signal line and signal ground with the key on/engine off to sweep the TPS, voltage should increase in a smooth uninterupted manner as the throttle is opened.
It may or may not set a fault. I have also seen spark plugs that were gapped to wide cause this type of hesitation problem.