Jump to content

1989 Jeep YJ Wrangler 2.5L TBI Vapor Lock


Recommended Posts

Greetings,

I have a 89 yj with the 2.5L TBI motor. And on hot days when I shut the motor off the fuel system will some times vapor lock. Iam looking for the pin out of the diagnostic connector on the passenger side fender under the hood. My hopes are  that there is a way to turn on the fuel pump  and keep it on to re prime the fuel system by jumping  a wire in the diagnostic connector  (Molex connector). Then once the fuel system has been primed remove the jump and down the road I go. 

 

I have several  friends that have done this with a GM 350 tbi motor and it works pretty darn good. 

 

Thanks you in advance for your help.

Brett77

Link to comment
Share on other sites

  • Administrators

Brett77...We can evaluate the diagnostic connector and see which pin triggers the fuel pump, but before taking that approach, have you considered checking the fuel pressure and regulator function?  The TBI unit operates at 14-15 PSI, and if the PSI is off or not holding, this could be an issue.  A quick test for overall fuel pressure, if you're gauge testing the fuel pressure at the TBI port, is to pinch the return line (only momentarily, just long enough for a PSI reading!) and watch for a high spike in the fuel pressure.  Share the pressure readings.

 

If you haven't changed the fuel filter, do so.  TBI with a return line should not create vapor lock, that's the significant gain with a return pipe system.  Even the 4.2L BBD carbureted engines have a third/return pipe on the fuel filter, and though those systems operate at a relatively low PSI, the continual overage of fuel flow goes back to the tank;  fuel does not stagnate in the line near the carburetor, and therefore vapor lock is avoided.

 

I've not heard of TBI/return line systems vapor locking, though it's conceivable if the pump does not cycle.  Why is the pump not cycling with initial KEY-ON?  What's allowing or causing the pump to "park" and permit vapor lock?  We need to look at this...

 

Moses

Link to comment
Share on other sites

Moses,

Got the fuel psi gauge out and performed the fuel psi test and the regulator function test. 

I got 15 psi at the throttle body inlet test port, motor at idle. And the regulator function test with the momentary pinch 17 to 20 psi depending on the length of the momentary pinch.  Filter changed

 

I need to give you more info.

The vapor lock never happens while driving down the road and only in the summer time. 

My jeep is not a daily drive. It's locked, lifted, geared on different axles and on big tires. I live and wheel  AZ. Gets hot here. Imagine wheel in a canyon where your ground speed is never gets above 7mph. Somebody gets hung up on a obstacle and it looks like it could take awhile. You shut the motor off. Say 15 to 20 minutes later the guy that got hung up is free and moving. My jeep starts right up, but travel 5 to 10 yards, the vapor locks starts happening. If I let it sit motor off for 30 to 50 minutes it will start back up and runs.    I don't want to be that guy that everybody is waiting on. 

 

The fuel pump does not park I can hear it running.  I think that traveling at low speeds in a canyon with little or no air flow across or through the jeep is a problem. I shut the jeep off it falls victim to heat soak. Fuels not circulating and boils sitting in the line.  My Vapor lock only happens in the summer.

 

I can also tell you that the guys in TJ's with the 6 cylinder MPI motors  have problem with this too. Not to the extent that I'm having, but it is happening.

The guys running the GM 350 TBI. They got around it by triggering the fuel pump motor off via the diagnostic port, to prime the fuel system. Works slick.

 

And well, there is the crap fuel that the powers that be have forced upon us.  

 

Anyhow Appreciate and looking forward to your input.

Brett

Link to comment
Share on other sites

  • Administrators

This is very helpful information, Brett.  Curious how triggering the pump with a switch is any different than when you hear the pump running normally? 

 

If this is vapor lock, you need a means for purging the input/pressure fuel line and TBI unit.  The fuel is stopped against the regulator.  If you're experiencing vapor lock, and it does sound like that's what is going on here, this is like a fuel bowl in a carburetor, and either the bowl or the input line is hot, causing fuel percolation and vapor lock.  Keep in mind that vapor lock may not be at the engine, it could be anywhere from the tank to the TBI unit if there is sufficient heat near the fuel lines.  The pump pressure does sound normal, and if there is no restriction in the return to tank flow, and if the pump is cycling on, the system should be functioning okay.

 

A true cure might be a manually switched bleed from the test port on the TBI unit to the tank return line.  Using a manual (electric solenoid) fuel switch, the line would open and drop the TBI pressure to zero.  Turning the key on, the OE pump would trigger and flow fresh fuel through the TBI unit that would pass through the custom bleed line.  Shut off the manual bleed line valve, and the pressure at the TBI unit would jump to regulated pressure.  Experiment with how much fuel should cycle through in order to eliminate the vapor lock condition.

 

The bleed line would need to "Y" (no restrictions here!) into the return line to the tank, and no fuel would be lost.  This bleed directly from the TBI unit would quickly move any hot, stagnated or vaporized fuel.  Shut the bleed line's solenoid valve off, and the pressure would promptly build back up at the TBI pressure regulator—with fresh, cooler fuel available. 

 

Caution:  The manual switch valve would need to be fuel grade and rated for more than the pump pressure (which is 17-20 PSI rated)—at least 30 PSI would be smart to prevent risk of bleed-off or leakdown when the valve is shut off.  (Unless you use a 100 PSI rated solenoid switch, do not pinch the return line with the solenoid switch is place—pressure can spike quite high!  Remove the bleed line and switch when performing a return line pinch test or testing fuel pressure at the TBI unit.)  When the manually switched valve is shut off, there should be no pressure loss at the TBI unit. 

 

I'd look at the Summit Racing catalog for ideas on a fuel cut-off solenoid.  Use EFI rated hose and Euro style EFI clamps with appropriate gasoline fittings.  Actual pressure would be somewhat low if there is no restriction in the return line, and the flow/volume is adequate.  I'd still use high pressure EFI hose and avoid heat sources in the routing.

 

You'd be in charge of the purge cycles via a manual electrical switch at the dash.  A brief opening of the bleed line should be enough to eliminate stagnation.

 

If you decide to do this, there's no need to mess with ECU connectors, pins or the OEM pump triggers...The TBI unit has an accessible test port that could provide fuel flow and purge the system. 

 

Make sense?  If so, please share photos and results!

 

Moses

Link to comment
Share on other sites

Hey Moses,

Liked the suggestion on the grade of parts and fuel psi rating.

You asked why I wanted to trigger the fuel pump to run.

When you turn the ignition key to the on position ( not starting the motor). The fuel pump will run for ~3 seconds and then the fuel pump parks.

My question is. What parks the fuel pump?

I was under the impression that.

Key on (no crank). The fuel pump runs for ~3 second and parks.

ECU does not see the motor crank (via Crank sensor. There’s not cam sensor on my motor) ECU says motors not cranking there’s a problem park the fuel pump.

So if this is true = Doing the by-pass solenoid valve from the supply to return and using a ignition key turn will only run the fuel pump for ~3 srconds a key turn. The ECU will park the pump because the crank shaft is not turning. Straight up. This really is not what I’m looking for.

So with all that being said.

I was hoping you might know somebody that has the Diagnostic connectors (cause there’s 2) pin out.

And maybe you do??

I have posted this on 4 jeep forums, 1 international off road forum, 2 Overland forums and nobody known’s answer.

Yesterday I had a bubby had me a Haynes manual, the YJ series published 1992. With electrical drawings. I was able to figure out with an ohm meter and the drawings what pin triggers the fuel pump. I was really hoping you could confirm it before I put power to it. I got scared.

I have been a lurker for awhile, really enjoy 4wd Mechinix Magazine

Once again appreciate your time and input

Brett

Link to comment
Share on other sites

  • Administrators

Brett...I understand your approach.  Am still questioning whether simply bumping fuel pressure against the regulator will break through vapor lock unless there is a return of fuel to the tank in the process. 

 

Respecting your intentions here, you will find the factory wiring for the fuel pump and ECU below.  Pay close attention to whether leads are positive source or negative source.  ECU and PCM leads often furnish the ground (not the "hot" or positive current) to devices that are KEY ON powered.  The ECU may be the ground source for a D.C. device, and the device has continual (KEY ON) 12VDC power that needs a ground for the device to operate.  Know whether leads are negative or positive before splicing and using your manual switch:

 

Jeep YJ 2.5L TBI Fuel Pump Wiring Diagram.pdf

[schematic #9 is at end of sequence.]

 

Note that you can either pull from the pin at the ECU or tap into the wiring to the pump power lead directly if desired.  All of this is color coded for identifying wires...The ECU lead to the pump itself could be a splice.  I'd advise use of a fuse and an inline relay between your manual switch and the pump lead.  You can source voltage from the ignition circuit to the inline relay if you want "KEY ON" (only) for your manual switch power.  Or, if you want to power the pump without activating the key, you can use a direct battery power source to power the pump via the manual switch, fuse and inline relay.  This would allow priming with the KEY OFF and ECU off as wellMake sure you turn off the manual pump switch after priming!

 

Look over the PDF schematics.  It's not complicated.  I'd prefer splicing away from the ECU, the ECU is less accessible than the pump wires.  With some thought, you can make this work without a lot of fanfare.  You're simply trying to complete the power circuit to the fuel pump via your manual switch.  Make sure voltage does not feed back to the ECU and damage it.  Isolate the switch signal with a relay and a separate power supply source if necessary.  A common "Bosch"-type relay can be used to either complete a power (+) circuit or a ground (-) circuit.

 

Let us know how this works.  If you need clarification on the current flow, please ask.

 

Moses

Link to comment
Share on other sites

Thanks Moses,

 

I really appreciate your feed back. And the attached prints have helped out a bunch. I was missing a couple of the pages in the manual I have. The complete set of the fuel pump circuit that you sent has helped a bunch.  

 

Thanks again for your Help.

Brett

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...