1990 lx 5.0 falling on its face

dillstang · May 4, 2010

I have a 90 lx 5.0 when i get on the car (full throttle) at 4500rmp it falls on its face and jerks, like im pumping the gas. this happens in all gears. If i gradually push the gas and come up to speed pass 4500 rpm then it doesnt do it. I just upgraded the engine with these upgrades- specs- holly systemax upper/lower intake, comp xe276 cam, noisy gear drive, flowmaster exhaust sierse 40 flowmasters with exhaust cutouts, mac shorty headders, 19lb injectors, stock tb, bbk 76mm mass air. Before i upgraded the engine, it ran horrible- when i got on the car it would for no reason go to idle and then run real choppy and gain power back. Im at a loss and need help/ My buddy has a completely stock one and took the throttle sensor off and tried that, but still no luck. tried stock mass air and it ran like it did when i got the car(horrible)

Haus · May 5, 2010

what size rear gears are you running?? and modifications to the heads/valve springs done. what size rockers are you using?

dillstang · May 5, 2010

I have 373 gears, heads are stock, put behive springs when i put the cam in, lifters and rockers are stock

SScott · May 6, 2010

First, I know many people like them, I love the sound but keep an eye on the noisy timing gears, I have had 2 of them fail before deciding to never go back.

I would look into the fuel pump. I have had the same thing happen and it ended up being the pump. If you can run a remote guage, even if it is zip tied to you wipers, that is an easy way to tell. If not the pump, what about spark? Look inside the cap for burned terminals and check the rotor. And check all the wires for arching.

Hope you get it going.

dillstang · May 7, 2010

Fuel pressure guage says 40lbs, so i dont think im loosing fuel pressure, when i did the upgrades, i had to change the ignition module and stator/pickup in the distributor. I checked the terminals and found a couple with a little bit of currosion(not much) so im going to clean them. I put all new spark plugs in and have racing plug wires. I also messed with the timing today and took it out again and not it does it at 4000rpm instead of 4500rpm, so i changed it when i messed with the timing... im stumped. keep trying things until i get it fixed. thanks

stang-geoff · May 7, 2010

is it reading 40 psi when you recreate the symptom?

SScott · May 7, 2010

hmmm, I don't know man, there are some really smart guys and gals here, I am hoping someone chimes in.

Haus · May 7, 2010

are you sure you have the right cam for your heads?? I could of sworn I read that you need certain modifications done to you stock heads and certain components to make the XE276 cam benefit you

RatStang · May 7, 2010

Pull codes
Spray engine start fluid around cold, running engine and look for vac leaks.
Check plugs, wires, cap, and rotor.
Check/replace TPS
Check/replace coil
Start car, warm up, disconnect MAF. Idle should change noticably. If not... clean MAF, chase wires, or replace MAF.

If wires look good but a year or more old... replace em anyway.

Let us know what you find.

WHT306 · May 7, 2010

Try swapping your distributor, the pick up coil inside goes bad causes similar issues. I have had it happen starts out with high rpm under load then gets worse and does it through all rpms at wot.

nickmav70 · May 7, 2010

change your plugs and fuel filter and see what happens... mine was doin the same thing and that fixed it

I candy · May 9, 2010

my 81 6 cyl did something like that cause of a clog catalitic.

dillstang · May 11, 2010

its got new wires, new stator in distributor, new ignition module, new coil, new tps. Im going to put a code reader on it and see what that says. the xe276 cam, I called comp cams and told them what i had, stock bottom end and heads, and they said that cam would workand i didnt have to have the heads machined. Fuel pressure is Between 32-40lbs all day, even when this happens. Only thing i havent tried is the throttle body. egr valve. I guess thats next, everything else i have changed. thanks guys ill keep ya posted.

StreetDreamsGT · May 12, 2010

dillstang said:
its got new wires, new stator in distributor, new ignition module, new coil, new tps. Im going to put a code reader on it and see what that says. the xe276 cam, I called comp cams and told them what i had, stock bottom end and heads, and they said that cam would workand i didnt have to have the heads machined. Fuel pressure is Between 32-40lbs all day, even when this happens. Only thing i havent tried is the throttle body. egr valve. I guess thats next, everything else i have changed. thanks guys ill keep ya posted.

Like Ratz said the best place to start would be to pull the codes, as long as you pass you can cross out the sensors and start looking for a mechanical problem.

dillstang · May 20, 2010

Took it to a speed shop, they told me it is the computer. I have a stock chip and i need one built for my car and dyno tuned. They priced $525 to build my chip install it and dyno tune, and give me the specks from the dyno. Guess thats the route im going. I have offically named my mustang..."GOLD DIGGER" lol

Strype · May 21, 2010

I had that problem as did WHT 306 and it was the Dizzy. You have replaced everything in the dizzy. So next I remember that I had the same problem with fuel pressure. You just said that it runs 32-40 lbs all day long... It should be running 40lbs all day long from what I have experienced. As far as I can remember anyway...

You are doing one of 3 things. Running out of fuel, out of spark, or out of air at 4,500 rpm. Air would be at any rpm. Fire, unless its the Dizzy (or plug gap) would be at any rpm. My gut tells me you have a fuel delivery problem (which can be caused by the computer).

A) Do you have an adjustable fuel pressure regulator, I assume you have a fuel guage.
B) When you adjusted the fuel did you do it with the vacuum line off and crimped?

If yes to all, have you messed with fuel system other than that?

I am not trying to drown out anyone else here cause I owe my Mustang to these guys, but like I said I'm thinking fuel. A vacuum leak would make the car idle crappy. A new PIP/cap/rotor button should take the dizzy out of the equation. The new TPS set below .99 should take air out of the equation.

The only thing left, or at least in a general perspective should be fuel.

Vinnietbird · May 21, 2010

Mine was acting the same way and I ended up having to swap the fuel pump.Not to say that is your problem,but,for the Sport,the fuel pump couldn't keep up.

dillstang · May 25, 2010

After talking to the speed shop, I decided to put the factory mass air back on(thinking an air problem)... It no longer jerks, i can run it pedal to the floor and doesnt miss. Only thing now it hickups in 5th gear and i smell gas. Doesnt do it all the time but it cuts out every once in a while. Its deff an improvement from earlier. But still something that bothers me and i want it fixed.

dillstang · Jun 3, 2010

PROBLEM FIXED

ok, after trying everything i could i finally found the problem, I wasnt even looking for it. I had a valve cover bolt head break and had to take the valve cover off. In order to do so i had to take the intake off, i found the intake gasked broke and wasnt sealing the ports, so gas and air was getting into other ports causing all my problems. I made a new gasket and sealed it with rtv and that fixed all my problems with hessitations/ hickup/ falling on its face. Thanks to everyone that threw out ideas, sometimes you find soultions when ur not even looking for them. It still goes to the dyno to get new performance chip and dyno tuned, just make it run even better!

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