need advice (speed density)
 

well i got my motor in today and it runs pretty good but i cant get it to idol very good i do have a bigger cam, and the car is set up with speed density, i took the tps sensor off and just set the idol up high to get it to run, the car bucks and does not like to run under 1500 rpm, is this hurting the motor at all with the sensor off, it kind of stutters under a load when at low rpm. also the throttle response is way off, i hit the pedal and it hesitates, it is really weird, if anyone has any ideas i would greatly appreaciate it thanks alot!

My SD car wouldn't run at all with the TPS disconnected. The TPS is vital to a SD car. SD has no way of measuring airflow directly.

What cam do you have, and what are your other mods? Have you adjusted fuel pressure?

9 TIMES OUT OF 10 A SPEED DENSITY CAR WILL HAVE A SURGE AT IDOL WITH A BIGGER CAM. EVERY CAR IVE SEEN WITH SPEED DENSITY AND A CAM SURGED. THERE MIGHT BE SOMETHING THAT I DONT KNOW THAT COULD HELP THE PROBLEM BUT I THINK THAT IT IS GOING TO SURGE NO MATTER WHAT.

It surges because the computer is injecting a specific amount of fuel (it thinks) @ 39psi. It goes on the needs of the stock cam/engine combo. If you get more than about 20% out of wack, the computer is still injecting like it's stock, but that amount of fuel is no longer adaquate, so the engine begins to die. When that happens the computer tries to compensate, which it does, and overcompensates at that causing a rev, back and forth, so on and so on. The condition is only made worse by the conflicting information from emissions sensors and engine sensors to the computer.

The way to get rid of this problem is to set the fuel pressure higher than stock. When you do that, more fuel is actually being injected into the engine than what the computer thinks, and more air is going in than what the computer thinks so the ratios even out. Setting the idle up also helps this problem, but 1500rpms is a little higher than what I'd want to set it to.

You try to adjust your tps at 1.10 volt to make the mixture richer

I can get the car to run at about 1100 rpm steady, without the tps hooked up i just bumped the idel screw up a few turns, i did have the fule pressure turned up but it liked to load up. i really cant figure out why my throttle response is so bad. i hit the pedal and it does not rev right away it bucks then goes,, anymore advice would help the cam is just alittle bigger then the FMS e cam.

What am I missing here? The ECM needs a signal from the TPS in order to know what to do. Saying you don't understand why it isn't responsive with the TPS disconnected is like saying "I don't understand why I'm soaking wet", while you're climbing out of the pool. EFI isn't like a carbureted engine. The throttle doesn't work the same way. With a carb, it was appropriately referred to as the gas pedal because as you opened the throttle, the engine vacuum and the air pressure would suck fuel from the carb into the venturi, and give you gas. The throttle forced the fuel. With EFI, there is no gas anywhere near the throttle. In fact, when cranking the engine to start it, if you hit WOT, the fuel will be completely shut off. The ECM controls the fuel delivery, and the only way the ECM knows what the driver is asking for is via the TPS, which, BTW, should NEVER be adjusted above .95VDC at idle. You seem so confident that it should run great without the TPS, that I figure I must have missed some part of this. Please explain it to me.

~Chris

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Retired Moderator
MustangNet

My site: JimPorterRacing

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14.28@103.0 on a G-Tech Engine: Steal Mounts, Under drive pulleys, K&N, Home ported Lower Intake, No power steering, No A/C
Drive Train: 3.55, World Class T-5,
NOW it is time for Real power STUFF I Have and will put on after I get everthing: Mac cold intake, 70MM TB, Valve covers (COOL ONES)255 Fuel pump NOS Dry 75HP.
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I had the same problem and tried the things you have and eventually ended up with an '89 mass air harness and sensor. The problem was then solved. You could also get a custom chip!!

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'88 Mustang GT-GT40 Iron Head Engine, Kenne Bell SC/FMU, AOD
www.geocities.com/rayscopes/RaysStang.html

I had the same problem and tried the things you have and eventually ended up with an '89 mass air harness and sensor. The problem was then solved. You could also get a custom chip!!

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'88 Mustang GT-GT40 Iron Head Engine, Kenne Bell SC/FMU, AOD
www.geocities.com/rayscopes/RaysStang.html

I'm just going to go ahead and tell you exactly how to fix the problem.

Buy an adjustable fuel pressure regulator, reconnect your TPS and set your fuel pressure up.

Oh, if that doesn't work, I'll buy the fuel pressure regulator from you.

You can try increasing your FP with the regulator if you want but I did and it didn't work for me.

I installed GT40 irons, intake, headers, roller rockers, etc. and had the surging. Do to an engine fire (due to a fuel line which wore through due to contact with the frame), I rebuilt it with the '89 harness and MAF. After this I never had the surging problem again. Although there were many things I probably didn't try to cure the surging, the FP regulator is something I did. Sorry for the bad news, this is only my experience, maybe it will work for someone else.

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'88 Mustang GT-GT40 Iron Head Engine, Kenne Bell SC/FMU, AOD
www.geocities.com/rayscopes/RaysStang.html

well i do have a FPR, PKRWUD, i did not say i seemed confident that it should run with the sensor off, i just said i could not get it to idle with it on. but i had the fuel pressure way out out of wack at the time i was trying to get it to run right. so anyway i threw the sensor back on and played with the pressure for awhile and i got it set to where it runs way better now. just a little hunt for idle but i can live with it, thanks alot guys for all the help

Awesome, the hunt may be a little of the computer relearning. Even my stock SD car occationally tries that crap. Rarely, but sometimes.

I'm glad it's running okay for you again.

What was the fuel pressure at?

I'm running speed density and i've done some airflow mods. There is another tool that you should get to make it run perfect: a rich lean gauge. This gauge will tell you everything and it will a lot easier to tune.