Why won't it start?

[stranger_5oh]

I'm in the final stretch of my swap and had the truck running. Ran great until I test drove it. It started bucking and spitting under a load so I limped it a few blocks back to the house to check things over and mess with the timing. Pulled the jumper out to put the timing light on and it died. Now it cranks but won't start. I've got fuel and spark. I checked the voltage on the TPS which is good. Swapped ou the TFI with a spare and it didn't change anything. I know it will start run without the MAF sensor but run like crap. Not sure if the MAP/BAP sensor (which ever doesn't have a vacuum line) being bad would cause it not to start. Pretty sure I've checked everything. If someone could please help me it would be grateful. TIA

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[RonD]

I assume you have a V8 since this is a V8 section but "V8" engine covers alot of ground, lol

Ford V8, I assume so, since TFI was mentioned?

302?

Carb or fuel injection?

On many older ignition systems there are 2 power paths for the Coil, when engine is running the coil is powered with 8volts so it will last longer, battery power passes thru a resistor before it gets to the coil "+" terminal.
But when starting you want the hottest spark you can for cold engine, so there is a separate path to send full 12volts to coil's "+" terminal while engine is cranking(starter motor engaged)
If this second path is disrupted engine won't start.
If you want to test for this put a jumper wire from battery "+" to Coil "+" and try to start

Best to give more details of the engine and Ranger year.

Speed Density systems used MAP sensor, these were older and couldn't adjust to engine changes well.
MAF systems started in the late 1980's for some engines, these were better systems but the two are not interchangeable.
A computer with Speed Density software can't be used with MAF sensor, and visa versa.
You can do conversions but that's another post, lol.

 

[stranger_5oh]

I assume you have a V8 since this is a V8 section but "V8" engine covers alot of ground, lol

Ford V8, I assume so, since TFI was mentioned?

302?

Carb or fuel injection?

On many older ignition systems there are 2 power paths for the Coil, when engine is running the coil is powered with 8volts so it will last longer, battery power passes thru a resistor before it gets to the coil "+" terminal.
But when starting you want the hottest spark you can for cold engine, so there is a separate path to send full 12volts to coil's "+" terminal while engine is cranking(starter motor engaged)
If this second path is disrupted engine won't start.
If you want to test for this put a jumper wire from battery "+" to Coil "+" and try to start

Best to give more details of the engine and Ranger year.

Speed Density systems used MAP sensor, these were older and couldn't adjust to engine changes well.
MAF systems started in the late 1980's for some engines, these were better systems but the two are not interchangeable.
A computer with Speed Density software can't be used with MAF sensor, and visa versa.
You can do conversions but that's another post, lol.

I have a fuel injected 5.0 in a 91 ranger. The engine, ecu, and harness came from a 91 mustang.

I have checked voltage to all sensors, coil, and the tfi. all of which have correct voltage going to them and have gone as for as to testing each one to make sure they are working properly. checked spark with spark tester and that's ok.

Checked fuel pressure as well and getting 40-45psi as required.

with everything checking out OK there is no reason why it shouldn't start. Going to change the plugs and wires just to rule those out and see what it does.

did I miss anything? Lol

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[RonD]

No sensor except the hall effect sensor in the distributor can cause a no start.
That sensor sets the pulse(PIP) for spark and fuel injection timing.
EDIT: I take that back, lol, the TPS sensor can send the computer 5volts by mistake, this will cause the computer to enter "Clear Flooded Engine" mode which will shut off fuel injectors, so unplug TPS and see if it starts , if not then problem is else where

If you add fuel manually to the intake will it start?

If so then fuel injectors are not working.
TFI module sends the PIP signal to the computer telling it to start the fuel injectors.
Computer sends back spark timing "updates" to TFI module on the SPOUT wire, the one you disconnect to set base timing without computer interference.

The PIP signal wire going to the computer is the important one.

You can also crank engine then pull out 1 spark plug, it should be wet with fuel if injectors are working

 

[stranger_5oh]

No sensor except the hall effect sensor in the distributor can cause a no start.
That sensor sets the pulse(PIP) for spark and fuel injection timing.
EDIT: I take that back, lol, the TPS sensor can send the computer 5volts by mistake, this will cause the computer to enter "Clear Flooded Engine" mode which will shut off fuel injectors, so unplug TPS and see if it starts , if not then problem is else where

If you add fuel manually to the intake will it start?

If so then fuel injectors are not working.
TFI module sends the PIP signal to the computer telling it to start the fuel injectors.
Computer sends back spark timing "updates" to TFI module on the SPOUT wire, the one you disconnect to set base timing without computer interference.

The PIP signal wire going to the computer is the important one.

You can also crank engine then pull out 1 spark plug, it should be wet with fuel if injectors are working

I check to see if it would start without the tps. It won't so I went ahead and changed the plugs, wires, fuel filter, and put 10 fresh gallons of gas in it. Still cranks but won't start. I also tried using some carb cleaner to see if it would start that way and still nothing.

Went ahead and checked spark again and I've got good spark. Checked fuel pressure and its reading 20-24 psi. I'm going to change my fuel pump again to see if that helps even though I changed it at the beginning of my swap.

Is there a way I can check to see if the tps to see if it's sending a 5v source back to the ecu like you were talking about?

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[RonD]

TPS has 3 wires, top wire is 5volts from computer, center wire should be just under 1volt when throttle plate is closed, and above 4.5v when throttle is wide open.
Lower wire is Ground,
Key must be on to test, use a sewing pin to pierce wire being tested.
But unplugging means it isn't the problem, unless wires are shorted in the harness.
And if adding fuel manually didn't get engine to fire then it isn't TPS.

You need 3 things for a cylinder to fire
Spark, at the right time
Fuel, at 14:1 air:fuel mix
Compression, above 100psi

Without compression fuel won't get hot enough to ignite.

Ether(starting fluid) has a lower temp ignition point, it can be helpful to diagnose no start.
Not sure about carb cleaner/degreaser.
But NEVER spray anything in through the MAF sensor, it will get damaged.
Pull off power brake booster hose or PCV hose from intake and spray it in there, then replace hose.

If you have a timing light, hook it up to #1 wire and see if spark is happening near TDC mark on crank pulley, it works even with only starter motor turning crank.

You can turn key on and off a few times and then see what fuel pressure is.
Ford computers only run the fuel pump for 2 seconds when key is turned on, it won't start again until RPMs are above 400, so after engine starts, turning key on and off a few times Primes the fuel system back to 40psi

 

[jeepfreek]

Did you get your truck running? Would like to know what fixed it.