MachFox346

Hey everyone. I've been stumped with an issue of mine. I have 91 lx convertible. It's a 5.0 w/AOD. I recently worked on the top end of the motor. From heads and cams to intake manifold. I installed new edelbrock heads and ford racing cams. The car ran great even with the stock manifold. My dad and I just installed the edelbrock upper and lower intake manifold. We also put on a new 75mm throttle body, 24 lb injectors, 75mm maf for the injectors. We replaced the air temp sensor, coolant temp sensor, and tps. Car runs great! But at idle it surges and almost wants to shut off. Does anyone have an idea? Thanks.

2k7gtcs

This may be a stupid question but does the MAF housing have the 24lb insert and not the one for the 19lb injectors?

MachFox346

Yeah everything is designed for the 24lb but I have a new fuel pump I haven't put in yet. Could that have anything to do with it?

2k7gtcs

You've checked all vacuum hoses for either splits or that they are all connected?

Not sure on the fuel pump. Maybe someone else can answer.

MachFox346

We checked all the hoses and replaces the worn down hoses.

skunk21

The stock fuel pump is undersized for sure once the h/c/i is done. I would also pull codes and see what you get ( these cars can have codes w/o flashing the check engine light). Also another huge myth, the stock MAF isn't a 19lb injector maf, the ecm controls the function. In laymen terms..Ford has a meter with a known air flow to voltage function (function is the math name for graph with a x and y axis OR f(x)=y). This function is known. Ford inputs this function into the ECM and call it a transfer function therefore ford can use any maf it wants. The aftermarket doesn't work with ford so the aftermarket sells a 'scaled' or 'skewed' maf/samle tube to trick the ecm when using a different size injector. The other option is to have a proper tune, ecm recalibration, done. End of guess work. It is possible because it has happened to others that the sample tube or maf is bad or has a bad "calibration" . you may want to contact thier tech dept about your issue for any help. I'm leaning on the maf causing the issue for starters

2k7gtcs

Another question?

Did you out headers on it? Have the O2 sensors been moved downstream.

This can cause a faulty reading and the computer dumps fuel in.

I think you might need to get a tune to take care of the issue. I've read numerous threads where this really helps on idle with a h/c/i and bigger injectors.

skunk21

Unless he installed long tubes this shouldn't be an issue. The fox uses 2 pre-cat O2s and can be moved.

I'm still leaning really heavy on the MAF. It happens more times than not guys have surging idle, overly rich issues or lean issues from them. I can get into a few long paragraphs on how the stock MAF isn't a 19lb inj. maf and how ford set up it and how the ECM uses a transfer function that converts MAF voltage to an airflow value and on and on. The aftermarket "recalibrates" or atleast tries to by modifying out put voltage ( this is one method) of the MAF sensor.The shape of the voltage airflow curve remains the same (hopefully), but it is shifted down by a ratio of old injector over new injector, in very quick terms. sometimes it works well sometimes it doesn't and there can be drivability issues.

again either take it up with thier tech or get a shop to do a "proper tune" for the injectors, get the air/fuel set proper and get her runner like she should. This is my hunch, I may be wrong.

Resolution78

A tune would fix the issue......been there done that

MachFox346

To be honest. We said we check absolutely everything. Somehow we never thought but it ended up being the timing. It was way off causing the car to wanna shut off. It ran great right after. It doesn't surge, doesn't shut off or hesitate. That's what did the trick. It's running excellent now. Thanks guys.