Help! Hesitation after cobra intake install

[shakermach]

Hi guys, my name is Richard. Im new to the forums but been around mustangs for years. I need help before I pull my hair out!!!

I have a '92 Lx that I just replaced the stock intake with a Cobra upper and lower, along with a 70mm throttle body and Procomp distributor. Car ran flawlessly before the swap and I kept the 19lbs injectors and mass air meter. The engine is completely stock, cam and heads... I have double checked all vacuum line connections and they are connected and in the right location, timing is set on 12 degrees btdc.

The car has a stumble or hesitation right off closed throttle or when you are what I call lugging the engine, like making a turn in second gear and getting back in the gas. seems only to be at low rpms just as you are cracking the throttle. If you accelerate on into it it goes away. I have done a couple of intake swaps before on previous cars and am ASE certified so I pretty proficient under the hood, I just dont know where to go with this.

Is it a fuel problem? Do I need bigger injectors, Adj Fuel Press regulator?

Thanks in advance for any advice

 

[BlownFiveLiter]

Do you have any codes? There could still be a vacuum leak somewhere, possibly with the intake gaskets, or the upper to lower. I'm sure jrichker will be along shortly with more ideas to try.

 

[shakermach]

Nope no Codes at all. Very weird, Im like you I think a possible vacuum leak. Im gonna change the orings on the injectors today and retorque the lower intake bolts.

This setup should be fine with the 19lb injectors and stock regulator shouldnt it?

 

[Kdubslugga]

You changed the throttle body, did you set your Throttle Posistion Sensor to apprx .98 volts? If not, probably your problem

 

[darthcual]

Yes the stock injectors should be fine. I would start by looking at the throttle body since that seems to be where the trouble is coming from. Make sure the throttle cable is routed correctly and not hanging up on anything.

 

[jrichker]

Think about what’s happening - what tells the engine that the throttle has suddenly been opened? With SD, cars, there is only the TPS and MAP sensor. Usually a dump of the codes would show code 22 (MAP/BARO sensor), code 53 or 63 (TPS sensor). Mass Air cars do a little better by using the airflow change to help calculate the required air/fuel ratio.

Since the TPS is the most likely problem source, start there.
The TPS sensor is a variable resistor like the volume control on most cheap radios. We have all heard them make scratchy noises as you turn them up or down. The carbon element can wear and cause a drop out spot in the output voltage. This confuses the computer because it expects to see a steady TPS voltage increase as the throttle opens up.

Wire colors & functions:
Orange/white = 5 volt VREF from the computer
Dark Green/lt green = TPS output to computer
Black/white = Signal ground from computer

TPS troubleshooting steps:
1.) Use the Orange/white & Black white wires to verify the TPS has the correct 5 volts source from the computer.
2.) Use the Dark Green/lt green & Black/white wires to set the TPS base voltage. All you need is less than 1.0 volt at idle and more than 4.25 at Wide Open Throttle (WOT). You'll need a voltmeter with a 1 or 3 volt low scale to do the job.
3.) Use an analog voltmeter to check the voltage on the Dark Green/lt green & Black/white wires to make sure that it increases smoothly and doesn’t drop out or increase suddenly. Slowly open the throttle as you monitor the voltage reading, watching for sudden changes that would indicate a dropout or contamination of the resistor element.

MAP sensor troubleshooting.

The MAP or BARO sensor is pretty much the same sensor for both Mass Air & Speed Density cars. The main difference is where it is connected. Mass Air cars vent it to the atmosphere, while Speed Density cars connect it to the intake manifold vacuum. Its purpose is to help set a baseline for the air/fuel mixture by sensing changes in barometric pressure. The MAP or BAP sensor puts out a 5 volt square wave that changes frequency with variations in atmospheric pressure. The base is 154 HZ at 29.92" of mercury - dry sunny day at sea level, about 68-72 degrees. You need an oscilloscope or frequency meter to measure it. There is no way you can use a common cheap voltmeter or DVM to accurately measure the output signal. There are a few inexpensive digital multimeters that have a frequency meter built in, but they are not easy to find.

The MAP/BARO sensor is mounted on the firewall behind the upper manifold.

Baro or MAP test using frequency meter - run the test key on engine off. The noise from the ignition system will likely upset the frequency meter. I used a 10 x oscilloscope probe connected from the frequency meter to the MAP/BAP to reduce the jitter in the meter's readout.

If it is defective, your air/fuel ratio will be off and the car’s performance & emissions will suffer

Some basic checks you can make to be sure that the sensor is getting power & ground:
Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter.
Check the resistance between the black/white wire on the MAP/BARO sensor and then the black/white wire on the EGR and the same wire on the TPS. It should be less than 1 ohm. Next check the resistance between the black/white wire and the negative battery cable. It should be less than 1.5 ohm.

The following power on check requires you to turn the ignition switch to the Run position.
Use a DVM to check for 5 volts on the orange/white wire. If it is missing, look for +5 volts at the orange/white wire on the TPS or EGR sensors. Use the black/white wire for the ground for the DVM.

 

[shakermach]

I did set the throttle position sensor and am getting .85v at closed throttle and 4.5 volts at wide open throttle. I retorqued the lower intake and changed the injector orings and still does the same thing. I have disconnected the IAC solenoid and used the throttle stop set screw to set the idle to 800 rpms then reconected the IAC and still the same scenario.

Like I said the car ran perfect before i removed the stock intake and a day later with the cobra intake runs like crap.
You really think that the Map/Baro sensor would have just happend to go bad? Seems kinda unlikely....

 

[stykthyn]

Did you use a new fuel rail? I did this once turned out the fpr on the new rail was bad.

 

[MFE92]

The other variable is the distributor. It wouldn't hurt, or take too much time, to stab the stocker back in there just to eliminate the variable.

 

[shakermach]

Nope same fuel rail that came off originally. I thought I would replace with the stock dizzy today and if that didnt work then put the stock throttle body back on....hopefully one of these will be the culprit. I am using a Proffesional Products throttle body and have read mixed reviews on them. Maybe it is the problem.

Thanks everyone for all the advice!

 

[hsean]

keep us posted i have the same setup stock 88 roller motor with cobra upper and lower intakes.just in case i have any problems when this thing gets thrown in the bay.

 

[5spd GT]

Mess with your distributor. It is not your fuel and if you know there is not a vacuum leak at the hoses, then I believe it is your timing. It could be as simple as loosening up the distributor hold down bolt and just turning it a 1/2" at a time, then drive it and check it out.

 

[aar0s]

Mine was doing something similar to what his is, 88 with cobra intake and 65mm tb. I checked everything, changed the dizzy, all the ignition, no codes present, vac was constant. could not figure it out. Did give me a reason to fix some minor things but none got rid of the problem

One night i noticed that it was idling at about 600, maybe a little less, got to thinking that maybe the idle was too low. Jrichker suggested setting the idle, I pulled the line off the IAC and the car died, idle was way too low, bumped it up to about 675-725 with the IAC unplugged and lost the hesitation and stumble at low speeds. I think that it had something to do with the position of the blade in relation to the tps, loading up on fuel coming off that low idle.

Could you have spun the throttle screw when you had the intake and TB off?

 

[shakermach]

Here is the update....

I changed out the throttle body back to the stock one Friday and thought that was the culprit. I drove it for about 20 mins friday evening with the stock tb on it and it ran fine, no studder at all. So Saturday I put all the interior back in it, new carpet, recovered seats...looking nice So Sunday I get it out and guess what same thing, missing and studdering then all of of sudden cleared up and ran fine. So I go back home, put the 70 mm back on it and it ran fine with it. So I drove it again yesterday and back and forth it would run good then miss, then run good again.

Needless to say, I got home from work and changed the distributor out and bam couldnt get it to miss again. Drove it for about 45 mins and all seemed fine.

I had a buddy helping me when we put it together and he actually put the distributor in, but I noticed last night when I pulled the Procomp dizzy out that it had a steel gear, not a bronze. My stupid mistake for not paying attention when we originally put it together, but would this cause the studdering and missing that I was experiencing or should I look at the electronics on the Procomp dist? Irrergardless I will replace the gear with a bronze one if I decide to try the Procomp again but just wondering if that is my only problem with that distributor?

 

[shakermach]

After reading some more it shouldnt matter if I run a steel distributor gear with the stock roller cam does it? I took the ignition module off the Procomp distributor and had it tested and it failed twice but then passed like 6 or 8 times.

 

[2002BLGT]

Get a stock replacement distributor , and new harmonic balancer , set base timing at 10-12 degrees