Jump to content

Jeep YJ Wrangler 4.2L Fuel Supply Issue or Vapor Lock


Recommended Posts

Hello people, I've had a hard time traveling in my YJ. Everything was ok until I stoped by a grocery store.

The engine have been having some abnormal behavior, I've been smelling fuel when I shut off the engine and step out of the jeep, when the tank is not full (say 1/4 of the tank) and I open the gas tank cap it releives a lot of pressure.

If I let the jeep sitting overnight, next day will give a hard starting and I have noticed that the carb bowl seems to be empty, as I can see there are no squirts when the pedal is pumping.

Now, regarding the day it completely failed, it was starting and then stallin unless I give it a hard acceleration, then start moving and it would keep the engine running for a while then dies.

Started troubleshooting by checking the squirts in the carb. No squirt.

Disconnected fuel hose at fuel pump inlet. Completely dry.

Attached an electrical reciprocating fuel pump. Same behavior after a while.

Fuel filter replaced with a single outlet one to eliminate return line. Could get the engine running longer but sometimes carb was flooded and sometimes I heared the electrical pump noise which means it is empty.

I've decided to get back home with the single oulet filter setup and both fuel pumps connected in line. The engine runs great even in hard acceleration when overtaking other vehicles. But if I slow down in traffic I will hear the electrical pump is getting dry again.

Now at home, tryed a new fuel hose from an external gas tank to the electrical pump. Same thing.

Routed a new fuel hose to bypass mechanical pump and steel line over the engine. Same behavior.

I've tried removing gas cap with no difference.

I've rebuilt the carburator and noted the base gasket was just with two barrel holes and no grove on the sides to let vac flow from the port that goes to canister (I think). Replaced that with the oval hole and side groves. But I'm still getting the pump dry.

As this is my DD vehicle I'm in a rush to fix this problem asap. Any help will be highly appreciated.

Going to drop the gas tank right now to discard pickup tube clogging.

Link to post
Share on other sites

UPDATE:

After dropping the tank found that it is kind of collapsed from the bottom, put some heat with a heat gun and it straighten itself, but when it coola down it will go back to the same shape.

No clogging in the pickup tube.

Today I was thinking about the pressure when open the gas cap and thought that maybe the tank is not venting, I was right. The steel line to the canister is completely blocked. I've disconected the hose coming from the tee at top of the tank but now it seems like the fuel filter is restricted. I can't blow by when I plug the return outlet.

Is there any way to replace that filter with a Tee and a clear filter? This is because it is hard to get here in town.

 

Regards. 

Link to post
Share on other sites
  • Administrators

Alberto, you did a very thorough troubleshooting job on this problem.  Sounds like you're near a cure.  The factory three pipe fuel filter is actually an asset.  When the fuel volume exceeds the amount that needs to flow into the carburetor bowl, the third pipe on the filter directs fuel back to the fuel tank (if the line is not restricted).  This continual, excess return fuel flow is a good thing, as it prevents the fuel from "pooling" in the line or filter and getting hot from engine heat.  When fuel between the pump and carburetor gets hot, it can cause a classic case of fuel vapor lock.  Fuel stops flowing...

So, be sure that the return line to the tank is not restricted.  The mechanical fuel pump should produce enough fuel volume and pressure to create the excess that flows back to the tank from the third pipe on the fuel filter.

I would not eliminate the 3-pipe filter.  If you can only find a 2-pipe (inlet and outlet) fuel filter, make a "T" at the line near the carburetor/fuel filter location to simulate the 3-pipe fuel system.  That way, you can maintain the fuel flow back to the fuel tank, which will keep fuel cooler at the carburetor.

Make sure the float level is correct in the BBD carburetor and that the needle is not sticking in its seat...

Moses

Link to post
Share on other sites

Hi Moses, Ironically the first thing I did was to replace the fuel filter, assuming that it was restricted to the carburetor. After finding the gas tank vent line clogged I deducted that all the extra pressure from gasoline vapors was traveling backwards to the filter throught the return line, then the vapor lock occurred.

After disconecting the vent hose from the steel line that goes to the canister I've noted that the pump is not getting dry anymore and next issue was the New fuel filter, it seems to be defective, fortunatelly I was keeping the old one and after putting it back the problem is temporarily cured.

I will have either to replace or unclog the  vent steel line.

Is there any reason to avoid replacing it with rubber hose from the tank tee?

another question, I noted that the whool on bottom of the cainster is not there, is there any problem running the engine like that?

 

Thanks for your comments. Regards. 

Link to post
Share on other sites
  • Administrators
9 hours ago, Alberto_YJ said:

Hi Moses, Ironically the first thing I did was to replace the fuel filter, assuming that it was restricted to the carburetor. After finding the gas tank vent line clogged I deducted that all the extra pressure from gasoline vapors was traveling backwards to the filter throught the return line, then the vapor lock occurred.

Good call here, Alberto...

Quote

After disconecting the vent hose from the steel line that goes to the canister I've noted that the pump is not getting dry anymore and next issue was the New fuel filter, it seems to be defective, fortunatelly I was keeping the old one and after putting it back the problem is temporarily cured.

Good...These filters should be available, the 4.2L BBD carbureted engine was popular, and the filter fits nearly a decade of AMC and Jeep models.  Do you have the NAPA part number for the filter?  Is there a NAPA distributor in your area of Colombia?  That would be the easiest parts source in the U.S.

Quote

I will have either to replace or unclog the  vent steel line.

Is there any reason to avoid replacing it with rubber hose from the tank tee?

The issue with fuel hose is safety.  There is heat and the risk of chafing/damage on the underside of the vehicle.  By choice and safety requirements, the vehicle manufacturers use steel pipe to prevent risk of fire around gasoline or gas fumes.  I prefer steel pipe.

Here is an article I did on EFI and carburetor fuel pressure requirements for Jeep inline sixes.  In the article, you will find details on how I form steel fuel/brake grade pipe ends to accept a hose.  I also talk about proper hose types and EFI grade hose clamp needs.  You can use standard fuel/brake tubing with double flared ends and flare nuts to piece together (if not long enough) a replacement steel line to match the original.  Sections of pipes can be joined together with flare seat couplers to match the tubing ends.  If you need to tie into hose at each end of the long pipe, see my method for cutting off the flare ends from the tubing and re-forming it as "bubble flares" to prevent the hose from coming loose or possibly getting nicked by a sharp tubing end.

Note:  You can buy 25' rolls of fuel tubing without flared ends.  Summit Racing has examples of steel tubing in bulk:  https://www.summitracing.com/search?SortBy=BestKeywordMatch&SortOrder=Ascending&keyword=fuel tubing.  Note the tubing sizes available, match your tubing diameter.  You need a tubing cutter and flaring tool to complete the tasks shown in the article.

Look down through the article linked below.  If you cannot unclog the factory steel line, you will get insights into how to fabricate safe steel fuel line with safer hose ends:

http://www.4wdmechanix.com/Jeep-Fuel-Pressure-Requirements?r=1

 

Quote

another question, I noted that the whool on bottom of the cainster is not there, is there any problem running the engine like that?

Do you mean the foam mesh that the canister contains?  Take one of your quality photos and attach it.  I'll comment back...

Quote

 

Thanks for your comments. Regards. 

You're welcome, Alberto!

Moses

Link to post
Share on other sites

Fortunately the issue has been cured, had to replace the vent line, didn't find a steel one, so I had to put copper tubing instead of steel.

 

Regarding the canister please see attached picture taken from under the vehicle.

I still have an issue with the carb bowl getting empty after I let it sit over night. Any suggestion to fix this? (Return line from the filter is already on top, carb has been rebuilt - al gaskets replaced)

thanks.

IMG_4381.JPG

Link to post
Share on other sites
  • Administrators

Hi, Alberto...The canister looks typical with its waffled bottom grille; the foam looks in place, that's normal...

When the carburetor bowl bleeds down after setting, look for an internal leak within the carburetor.  This is usually 1) a check ball either not seating properly (possibly the wrong size ball) or  2) a check ball not in place at a level either below the float bowl or low in the carburetor.  Evaporation of all the fuel in the bowl, as you know, would not occur overnight.

I would check for a fuel bleed-off low in the bowl, like the accelerator pump's check ball, the check ball's weight or poorly adjusted accelerator pump linkage.  Refer to your carburetor rebuilding kit's parts diagram/schematic.  The bleed is from a passageway or a circuit that is lower than the fuel bowl...A leaking float needle would not drain the fuel bowl.

Moses 

Link to post
Share on other sites
  • 4 years later...

1987 jeep wrangler 4.2, ran good. Needed new skid plate as rusted thru. Found puncture in tank. New tank, new sending unit. New fuel pump.new carb. Gas return line clear from new 3 way filter to tank. Starts but gets up to about 1800 rpm in any gear and sounds like it starves for gas, dies. Then it will start back up. What is happening? Help!

Link to post
Share on other sites
  • Administrators

Harry...I would check the carburetor float level.  Could be set too low, not allowing enough fuel into the bowl.  If that's okay, check the fuel volume at the carburetor side of the factory 3-way fuel filter.  Pressure is not enough.  You need adequate volume

If you don't have the fuel pressure and volume specs, I can furnish them.

Moses

Link to post
Share on other sites

Thanks Moses, original carb I replaced with a rebuild as could not pass smog, it did this starvation thing so took to a shop as was i out of personal time to devote. They said it was wrong carb and took it back to supply house and exchanged. I picked it up and towed it to my place about 5hrs away and found new carb does same... But original did not. It is good until load demand at 1800 rpm for about 100' then dies.

What is my float setting to be? or do I just keep adjusting until flow sufficient. Is there a fuel pressure gauge I should have shipped as i am remote, yes specs and hook up location for pressure and volume would be appreciated. Just worries me that both the rebuild and new carbs were factory set, guaranteed, and both did this.

I notice that where both steel lines run to the tank the steel lines stop at a cross member just before the tank, hose clamps to rubber and  loops in a 360 circle about 4" diameter to go through the space between crossmember and the body and hose clamps again to fittings then  rubber on over to the sending unit. Does that sound standard? Could I loose volume in a 360 loop of 4" diameter, or pressure? The original tank I replaced because of leak where skid plate was damaged was plumes this way, I just hooked up the new steel tank and sending unit.

Link to post
Share on other sites
  • Administrators

Harry...Given the history with the two rebuilt carburetors, before playing with the float level, my first step would be a fuel pump pressure check and measurement of fuel volume/flow going to the carburetor.  The BBD carburetor is "busy", and setting the float level is not as easy as some carburetors.  If you do need to check the float, my how-to article at the magazine should be helpful:  https://www.4wdmechanix.com/Rebuilding-the-YJ-Wrangler-4.2L-BBD-Carburetor?r=1

Fuel pump pressure is tested with the fuel line disconnected from the carburetor and the filter return hose disconnected with a cap placed on the filter nipple.  This provides a straight through pressure check.  In this state, the fuel pressure check should turn up 4-5 PSI at the disconnected carburetor inlet.

The fuel volume check can be made with a combination pressure/volume tester, which routes fuel into a catch can.  I use a metal can and stay away from the engine fan and other moving parts.  This test is with the engine at a curb idle speed.  Running for 30 seconds should deliver at least 16 ounces (a pint) of gasoline to the catch can.  The factory follow-up for low volume (when pressure is normal) is the use of an auxiliary fuel supply at the suction side of the fuel pump, which bypasses the gas tank as the fuel source.  This helps determine whether the limited fuel supply results from a restriction between the tank and fuel pump.

Restrictions in lines to the pump or poor tank venting can create low fuel flow volume.  The loop you describe at the crossmember near the fuel tank sounds odd.  If you can take a few cell phone pictures of this arrangement, post them here.  We can make an assessment.

Poor tank venting is a common problem.  The tank has a complete venting circuit, including the rollover valve(s), the vapor or EVAP system and the gas cap.  Any misrouting of hoses to the EVAP canister, tank vents or filler pipe can create an air block that would restrict fuel flow.  You mention replacing the fuel tank;  any hose or pipe restriction could be involved.  Also, subtle troubles as simple as a loose hose connection between the suction side of the fuel pump and the fuel tank can reduce fuel draw.

If fuel volume is low despite adequate fuel pressure from the pump, I would check all hose routings, connections, pipe connections and the rollover valves.  If I understand correctly, the engine is shutting off completely?  You can then restart it?  That sounds like either a tank vent system issue or poor fuel volume to the carburetor.  These are some diagrams and exchanges that may be useful:

If you have a manual transmission, this is your vacuum diagram:

YJ Wrangler 4.2L Vacuum Diagram.pdf

Check the EVAP vacuum and other EVAP hose routings at the gas tank and fuel filler.  A poor gas cap seal can raise havoc and create fuel flow problems.

 

Moses  

Link to post
Share on other sites

Wow, thanks for the diagram, bought a Chilton's and it dosen't have one as good. 

I see that one vent shows from the canister, where does the other come from?

That is news about the gas cap seal, is doesn't seal good, this 1987 system is not simple like you would expect for the age. The flapper in the neck does not sit and the mouth is bent...I need a new neck fitting and cap... almost like someone tried to steel gas and got angry...did not expect performance problems from that.

 

 

Link to post
Share on other sites

 

 

Okay, thanks so much, will get on your suggestions immediately. 

I did notice the flapper at the mouth of the fuel tank filler may not be sealing as is bent a bit, does it and the cap need to seal, holding pressure? 

Sunday;

Cut some gasket material and added to cap to try to get a seat as can not find replacement. Bowl starved st 2800 rpm rather than 1800. I notice you mentioned Roll over valve which are at the tank vents. The new tank has bent tubes welded in the top, no such valve. What are they for what do they do? Do those vent lines actually pressurize the tank? Where does the other tank vent line go?

 

Link to post
Share on other sites

Fuel pressure is 4-5 lbs but flow at 30 seconds at idle is about 1/3 pint. Same if I bypass the filter. I assume the suction side of the new pump is okay, changed out old pump was probably good too as no change in performance when switched. Does that mean a restriction in supply from tank? Vents should be presurizing, right? Thus your cap sealing comment...

Link to post
Share on other sites
  • Administrators

Harry...You're getting close to the issue, it's obviously a low fuel supply volume.  This points to a restriction, air leak or pressure issue at the fuel tank.  A restriction can be anything from a clogged filter pickup in the tank (base of pickup pipe) to a kinked hose, possibly the circular loop you describe.  The pressure issue can be the EVAP system, which includes the fuel filler cap.  2,800 rpm is much better than 1800, so the cap gasket apparently helped.

A pressure-vacuum filler/gas cap has a two-way relief valve that is closed to atmosphere during normal operation. The relieve valve on your Wrangler will open when a tank pressure of 1.1 psi or a vacuum of 1.5 in/hg occurs.  When the tank pressure or vacuum normalizes, the valve in the cap returns to its normal, closed position.  This is part of the emissions/evaporation system.  The system and filler cap are closed to atmosphere and rely on the canister and tank pressure to operate properly.

image.png

I did some research on the 1987 YJ Wrangler fuel tanks.  If you have the smaller 14.5-gallon tank, there are no vent/rollover valves.  (See above.)  If you have the 20-gallon (plastic) tank, it has two rollover valves.  (See item #19 below.)  The 4.2L engines do not use a fuel pump in the tank, and the sender and pickup tube are different than electric fuel pump applications.  The 2.5L uses an electric fuel pump (Item #7 in both illustrations) in the tank.  The tank you describe is the 14.5-gallon illustration.

Did you install a new sender that has the pickup tube reaching to the tank's floor/filter as shown in assembly #4 in each illustration?  If you installed the assembly #3 style sender and pickup tube, that is the style for an electric in-tank fuel pump.  The #3 style sender has a short pickup tube that will not reach the tank floor and floor filterIt would not be submerged in fuel unless the tank is somewhat full, otherwise starving the engine and fuel pump for fuel...Your 4.2L with a mechanical fuel pump needs the sender and pickup tube shown in assembly #4, the type used with the 1987 4.2L mechanical, engine mounted fuel pump...Zoom-in for details.

 

image.png

With the 20-gallon plastic tank, the fuel tank pressure relief and rollover valve(s) 1) relieve fuel tank pressure and 2) prevent fuel flow through the fuel tank vent hoses in the event that the vehicle rolls over.  The valve(s) will prevent raw fuel from flowing through the rollover valve orifice and into the fuel tank vent tube in the event of a rollover.

In any EVAP system, there is a pressure loop that includes the fuel filler neck and gas cap.  The gas cap must function normally, which requires a good cap gasket seal.  Visualize that in order for the fuel pump to draw (by vacuum), there must be sufficient pressure above the gasoline in the tank.  If that pressure is not normal, or worse yet it is a vacuum (like from misrouted EVAP or fuel lines or insufficient venting), the fuel pump cannot pull fuel through its feed line.  This is why a closed system like EVAP with a loose filler cap or defective fuel cap vent can prevent the fuel pump from drawing its full fuel capacity.

To demystify this, you did not have the "carburetor problem" (which is actually a fuel volume/supply issue) with the old tank and sender.  Now you have it.  This suggests that you now have the wrong sender/pickup or the tank's venting system or EVAP hose routing is the culprit.  

Check into the sender/pickup part number that you installed.  See if the new sender is for the 4.2L with a mechanical (out of tank) mounted fuel pump.  Also, did you install a new pickup filter on the sender assembly?

My rule of thumb when a problem arises after replacing a part(s) is to consider the sequence of change:  Before the tank and sender/pickup change, no problem with fuel volume;  after the tank and sender change, a fuel volume/flow issue.  What, if any, is the difference between your original fuel tank and this one?  Did you install the 4.2L sender/pickup or a 2.5L type for an in-tank electric fuel pump?  The 4.2L sender/pickup has a provision for the fuel tank's pickup or floor filter, which is item #9 in the illustrations.

Moses

Link to post
Share on other sites

It was detail 4, the screen at the pickup is actually a cylendar screen nor flat like shown in #9, but all else is the same. 

I just hooked up the fuel line from the pump directly to the carb without the feed bypass to the tank or the filter, she runs, no starvation. Could the new filter be bad? Or is the feed back line to the tank taking all the flow and pressure some how?

Link to post
Share on other sites
  • Administrators

Harry...You could check the fuel pressure and flow volume at the inlet end of the fuel filter.  Attach your gauge and fuel Tee at the inlet side of the filter.  This would provide an accurate pressure and volume reading from the fuel pump without involving the filter.

Moses

Link to post
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...