tbi problems

doctordick · Jul 22, 2011

Put a new sock on the bottom of the fuel pump, you can't tell if they're bad, they're cheap and can cause exactly the problem you're describing. Took a few minutes to find this, MAP sensor should 1 - 1.5V at idle, 4.5 - 4.8V at WOT PIn C11 at the ECM, light green wire.

doctordick · Jul 24, 2011

Anybody got their junk fixed yet? :dunno:

Sure like to know what you found!

MUD MAN · Jul 24, 2011

I get off the boat on Wednesday and hope to have it figured out by the weekend! Thanks for all the info man! :dblthumb:

COLT · Jul 26, 2011

doctordick said:
Anybody got their junk fixed yet? Sure like to know what you found!

Yessir! Sock on the pump came apart and tore the pump up. Weak pump= no woopow! I put a bosch pump in it and she's right on now! Thanks for the help, i really appriciate it! :dblthumb:

RBaker · Jul 26, 2011

I have been working on mine and still have not got it perfect. I have almost determined that i have a intake gasket leak and it is sucking air internally at high rpms. But it still confuses me why it will only bust up if I stab it and not under gradual throttle. Also a new problem has came up when I start the motor while ideling the rpms rev up and down and never plains off. You can pat the throttle and it will idle at 900 rpms for a few seconds the revs back up where do I start with this problem. My rig performs good I am just anal about motor issues.

Elliott · Jul 26, 2011

Have you tried spraying around the motor with starter fluid to check for vacume leaks?

RBaker · Jul 26, 2011

Not yet the idea of spraying starter fluid around the coil worries me. Could it be my throttle body gasket.

sappmann · Jul 26, 2011

TBI are bad about sucking the gskt in take the bolts out and check

RBaker · Jul 26, 2011

Are you talking about the gasket between the tbi and the intake

sappmann · Jul 26, 2011

yes, they usually suck the gskt in where the pvc hose is on the front you can take the 3 bolts out and raise it up and see if the gdkt is messed up

doctordick · Jul 27, 2011

Quick check for a vacuum leak : monitor the Map sensor output with a voltmeter. normal range would be 1v to 1.5v at idle up to 4.5 v to 4.8v at wide open throttle. Measure at the light green wire on the MAP sensor. Higher than 1.5 volt at idle would indicate a vacuum leak. That said, vacuum leaks mostly effect idle and low speed performance. At higher RPMs vacuum decreases to almost nothing anyway. Without getting into a long technical explanation, fuel pressure problems kind of act ass backwards in a TBI system than in a carbureted system.

RBaker · Jul 31, 2011

I spent the day working on my rig and a few others. I changed a pinched fuel line that I thought was the problem but turns out not the case. I checked voltage at my map sensor and I only have .47 volts at idle. What could cause this a vacum leak or what? :wtflol:

doctordick · Jul 31, 2011

RBaker said:
I spent the day working on my rig and a few others. I changed a pinched fuel line that I thought was the problem but turns out not the case. I checked voltage at my map sensor and I only have .47 volts at idle. What could cause this a vacum leak or what?

Nope vacuum leak will cause the voltage to be too high. Rev the engine, voltage should go up. Disconnect the connector from the MAP sensor and check the voltage on the grey wire. Check for corrosion on the connector pins. It should be 5v with the ignition on. If that's good, check voltage between the grey and purple wires, should be 5v. Disconnect the battery lead and remove the big connector from the ECM, check for corroded pins at c11 and D2 light green and purple wires. (pin positions are marked on the back of the connector).. If all checks out, either the Map sensor or the ECM is bad. Try running the engine with the Map sensor disconnected, I think there's a default (limp) mode for this. Engine should run fairly normally. I'll try it tomorrow and let you know.

RBaker · Jul 31, 2011

I will try checking these voltages tommorrow.

doctordick · Aug 1, 2011

The ECM does have a default program, so just disconnect the MAP sensor and see if it runs normally. Since the 5v input is a pulse signal, you would need to measure these voltages with a "true RMS" meter or a scantool. That .47 voltage that you read would probably be OK if you were using a general purpose type meter. I think we may be getting led astray here. If it doesn't run a lot better with the MAP sensor disconnected, you'll need to move on to ignition and fuel pressure. Check timing and fuel pressure as explained farther back in this thread. Either way, you defiinately don't have a vacuum leak.

RBaker · Aug 1, 2011

I am going to unhook the map sensor and see if it runs better. If so would this indicate that I have a bad sensor or other problems.

doctordick · Aug 2, 2011

Had a thought today (hate when that happens) Is your IAC working? That could account for the weird idle and cause a lean condition at wider throttle opening. Foot off the gas, engine warm, does the RPM come back up when you put it in gear or start to drag the clutch ? If you have access to a scantool, you can operate the IAC with the engine off. If not, you should be able to watch it operate with the engine running by removing the air cleaner and spacer ring and looking straight down into the opening above the end of the IAC. Try it with the engine cold, the pintel should extend and then slowly retract as the engine warms up. That's an advantage and a disadvantage of the earlier computers, they can only tell if a part is shorted or open, not if it's just not working mechanically. That lack of diagnostic power is also what lets us get away with murder without reprogramming compared to the newer engine systems. Hope this helps, lemme know.

RBaker · Aug 3, 2011

I replaced the throttle position sensor and the idle air control valve about a week ago. Today when i was working on the jeep when I unplugged the map sensor the truck would idle up and hold a constant rpm. I checked voltage and had .5 volts to the grey wire and the same acroos the purple and grey wires. As soon as I plugged the map back in the engine throttled down for about five seconds the back up. I had a spare ecm so I tryed it and nothing really changed it still had the same issues. Could the coolant temp sensor be causing the engine to idle up when cold if so is it possible that I have a bad sensor since it it still idles up when its hot.