1987 4 cyl MJ Rough running condition

[jeepogeek]

Good day all.  I'm new to the club and an old dude at that.  I recently bought the '87 MJ knowing it didn't run and the owner said he ran the fuel tank empty and said it needed a fuel pump.  I thought how coincidental it was for that to happen.  So I decided to trace for power at the the fuel pump plug and it doesn't seem to be getting juice there.  I'm not that familiar with these vehicles so I'm asking for help with ideas on how to track it down.  I read a little bit here about fuel pump issues and the fact there isn't a fuse but there is a relay.  I can only see two  square blue relays under the hood.  can any of you guys help?  Here's a pic of what mess is under the hood. 

[Ωhm]

Remove both relays and look at the relay blocks.

Check Pin_4 on the relay blocks. The one with ORN in it, will be the Fuel Pump Relay. The one with PNK in it, will be the B+ Latch Relay.

[eaglescout526]

@Ωhm, ready for round two? Lol.

 

But welcome to Renix 2.5L. How the fuel pump system works and a great way to verify it works is put the key in and turn it to the start position but don't actually start the engine and listen for the fuel pump to prime the rail. If you don't here anything from the fuel pump then chances are its shot and needs to be replaced. The ECU triggers the relay except at the priming sequence. 

[Ωhm]

10 minutes ago, eaglescout526 said:

@Ωhm, ready for round two? Lol.

I'm ready. Lets do it.

[jeepogeek]

So, when you say check it at pin 4...how do I do that.  You mean for continuity?  and here's the deal..I am not going to change that pump until I know it is bad.  Lol.  I want power to the plug>

 

 

OMG?  Thanks for all the advice.  And here is the patient, ahaaa MJ before I bought it.

[Ωhm]

52 minutes ago, jeepogeek said:

So, when you say check it at pin 4...how do I do that.  You mean for continuity?

Check the color code on the wire that goes to Pin_4 cavities. This will let you know which relay is the Fuel Pump relay and which relay is the B+ Latch relay.

ORN=Orange

PNK=Pink

[jeepogeek]

Okay.  I'll see what I can do about it, and thanks.

[jeepogeek]

Okay, so is there a recommendation for which fuel pump to buy?  My MJ is a standard shift 2.5 l.  Thanks!

[eaglescout526]

A good reputable pump brand. I have a napa one in my truck that's still going strong. Bosch was the OEM pump but their pump has shrunk and won't fit without any modifications.

[jeepogeek]

Well, I removed the old fuel pump assy. and low and behold there was no strainer or sending unit attached.  I had already bought a pump and strainer from Car Quest, and now it seems no parts houses I've tried have the sending unit.  Do any of you know if I can even get a sending unit for the 2.5l engine?   This is what I pulled from the tank below photo.  Notice there are only 2 nozzles that go into the tank.  I saw an illustration on Napa site showing a sending unit with 3 nozzles.  Don't know if that would work.  Could someone please advise on this whole issue?  Thanks

[eaglescout526]

If you can find all the parts in your tank, send it out to be rebuilt. If you can’t then get a new one from MTS

[jeepogeek]

Okay thanks for your quick reply.  As it turns out I ordered one on Ebay.  Guess it's probably the same thing since it is the same price.  I just hope the pump will fit onto it OK.  Sorry I jumped the gun and sent a frantic message out, but I really appreciate your help.  Maybe after I get the thing running I won't be so frantic.  Peace brother.

[eaglescout526]

No worries. Been in your shoes a few times. It should fit, might have to modify it. Also @JMO413, doesn't the MTS units comes with a pump?

[JMO413]

You can order it with it without the pump.

[JMO413]

So I see the update took away the edit feature also. I meant to say you can order the sender with the pump or without. If I recall correctly it's about $60 difference.

[eaglescout526]

1 minute ago, JMO413 said:

edit feature

 

 

Its under the 3 dots in the upper right hand corner of your post.

[JMO413]

2 minutes ago, eaglescout526 said:

 

 

Its under the 3 dots in the upper right hand corner of your post.

Thank you I never would have found it.

[eaglescout526]

10 minutes ago, JMO413 said:

Thank you I never would have found it.

No problem. I had to edit something too and found it under those. 

[jeepogeek]

So now, sometime later, I checked the old pump that I removed from the tank by bench testing direct to 12v current.  And, don't you know the old pump works fine.  Now I ask you, what is the best way to trace power back to the pump plug?  And is it necessary to have someone turn the key while I'm probing the harness?  Is it common for the relay to fail?  Btw, I redid the ground at the tail light. 

[eaglescout526]

It is common for 30 year old relays to fail. It wouldnt hurt to replace it with a known good one just to see whats up. 

 

So a while back @Ωhm and I helped another member test everything on his 86 2.5L and traced the entire fuel pump circuit. 

Not sure how much info carries over from 86-87 as I own an 88 but I recommend giving this a glance and try tracing some of your wiring and see if you can find the short somewhere or lack of power flow. Also you probably have the C101 which is located by the emissions label on the firewall. It is highly recommended to clean that out when any power issues arise as it is the biggest resistance in the wiring harness for 87 and 88. 

 

Let us know if you need any help and we will do it all over again.  

[Ωhm]

30 minutes ago, jeepogeek said:

And is it necessary to have someone turn the key while I'm probing the harness?

Once you determine which Relay is the Fuel Pump Relay, remove it and jumper Pin_1 to Pin_4 on the relay block. This will supply B+ to the Fuel Pump at all times. No need for KEY ON.

[jeepogeek]

You are the MAN!!!  Thank you very much for the jumper trick.  It allowed me to see that there was power from the plug to the pump and there was a failed wire spade terminal at the pump.  I really appreciated everyones help.  I still need to install everything back into the tank and see if the truck will start, but I'm quite certain it should.  By the way, what is all this "B lock" and "B+" you refer to.  I'm not familiar with it.  But I presume you are a wizard at electrics based on your user name.  Thanks again, and I'll revert with the verdict on whether the truck runs. 

[Ωhm]

2 hours ago, jeepogeek said:

By the way, what is all this "B lock" and "B+" you refer to.  I'm not familiar with it.

"B+" is just my quick way of typing 'voltage from the battery positive terminal'. But I see what you mean since the vehicle is equipped with a "B+ Latch Relay". I can be confusing at times.

[jeepogeek]

So what does the other relay regulate?  Latch?  What is that?

[Ωhm]

1 hour ago, jeepogeek said:

So what does the other relay regulate?  Latch?  What is that?

Purpose of the "B+ Latch Relay" (aka "Power Latch Relay") is to reset engine idle speed for next ENGINE START. Relay stays LATCHED (activated) for 2-4 seconds after KEY OFF supplying voltage to ECU allowing ECU to reset idle speed.