Ok so I had a 94 Mustang 5.0 HO motor rebuilt and installed in my F150 with a crane cam, roller rocker conversion, headers, but the stock f150 intake manifold, throttle body, and fuel system. My issue is that the engine idle's fine until there is a load on the motor then it won't idle at anything less than 5k rpms. I replaced the idle air control motor thinking that could be it with no luck. I have checked the vacuum lines and they are all plugged in and I couldn't feel any leaks. the guy who did the swap said that I need to put an intake manifold and throttle body from a 94 Lightning on it and get my exhaust done and possibly have my computer re-mapped for the best running and power production. My question is what in the world is making the idle jump way up?

I have also unplugged the idle air control motor and that prevents the issue, but this is my second idle air control motor in two days. Please anyone who has done this swap or can help me out HELP ME!

James

 

What is your TPS set to?

 

I didn't know you could set it to something. How do you do it? I just bolted it up and plugged it in. The guy who did my motor didn't say anything to me about re-setting the tps or adjusting it. Any info is greatly appreciated. Also through searching I read that on a 92 F150 the firing order had to be switched, mine is set to what it was on the truck originally and it runs, but do I need to change the firing order to 1-3-7-2-6-5-4-8??

Thanks for the help

James

 

You should always check the TPS and see what it reads, but that's not important now, you just solved your problem. The firing order is determined by the camshaft, so if you have the camshaft from a Mustang in your engine, then that is the firing order you must use. If you're using the old firing order, only half of your cylinders are firing correctly.

 

I'm not using a factory mustang cam. I am using a crane cam, and a roller rocker conversion with roller lifters, but I'm thinking they were for a mustang and not a truck. But I'll switch the firing order and see how she runs just to be sure. Also to set the idle do I just disconnect the idle air controll motor then adjust the idle so it idles good then plug the iac back in and reset/check the tps voltage from there?? or should I adjust the tps first and then do the idle? One last question would the intake manifold from a 94 lighting work/fit on my 5.0L motor (my engine builder says that would work and help me get all the performance out of this motor) Thanks for all your help pkrwud you are truly a life saver.

James

ps I searched and found out how to adjust the tps.

 

Lightnings had 351's. 351's have a taller deck height than a 302, which means the intake manifold for a 351 needs to be wider. Think of the 351 as a capital letter V, and the 302 as lowercase letter v, and the intake manifold sits on top of the letter in each case (Vv). See how they would have to be different widths?

Reset your firing order, and you should be fine.

 

The Lightnin uses a Gt-40 2 piece manifold. If you had a GT-40 lower half for a 302 lying around, you could use the Lightning upper. You may not need to do anything to the intake. Like PKRWUD said, solve the known problem with your firing order and see where you stand.

Which Crane camshaft did you use? Did you use the Mustang heads or the truck heads?

 

I can't remember which crane cam it is, but I'll look around for the info. I am using the mustang heads. I'm going to reset the firing order and see how she does from there. I am also going to check with my engine builder to see if he has any mustang gt-40 lower halves that I can add a lighting upper half to. Then all I have to do is add the lightning tb and a cold air intake and then I should be halfway there. I'm going to have a 3" exhaust made as well to go straight from each header and out right in front of each rear tire. Hopefully then I'll be problem free.

James

Thanks again for all the help and please keep it coming.

 

A pair of 3" exhausts is probably too big unless you're running a very, very big cam with aftermarket heads and intend to turn beaucoup RPM. You need to keep exhaust velocities up to maintain mid-range torque. I'm guessing that 2 x 2.5 is going to work just fine in your application, possibly even 2 x 2.25 depending on your camshaft and your gears.