need some help with ECU? EFI relay? intermittent starting issues (1 Viewer)

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I've read thoroughly many different posts related to the issues I'm having. However I cannot figure out what the issue is.. if anyone can help I'd appreciate it.
have a 1991 fj80 4.0
it has had intermittent starting issues mostly in the morning. The CEL does not come on at the key turn
when the vehicle is not starting and the CEL is not present at the start, then the codes cannot be read by jumping at diagnostic box
when the vehicle is working properly the codes read 21 and 28.. upon inspection the o2 sensor has a wire pulled loose
i've replaced the EFI relay the first time the vehicle didn't start and it worked.

is there a chance the o2 sensor is causing the CEL to not come on at the start?
is it most likely a short?
i'm not the most compitant with electric but i can probably figure it out if someone is willing to help.

super bummed about the truch being down, i'd like to feel more confident on the trail with it.
 
Without a CEL on when key is turned to run before start it will never start as the ECM is not getting power. Most common cause is failed fusible links coming off the battery. $15 and a few minutes to replace should be your first setup. It's possible that something is shorting to blow one of the links but a lot fail because of age and messing with batter cables.

A O2 sensor will not cause a no start problem but if the wiring harness to the sensor is damaged and shorting to ground can cause the fusible link to blow.
 
IMHO, stop reading and get yourself two fusible links. One to install and one for the glovebox. If that's not it you're out $30 for preventative maintenance and trail insurance.

Unless...

Did you recently (or ever) attempt to disassemble the VAF AFM? It's the air meter on the main intake hose upstream of the throttle body.
 
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I did the 3 tests for the fusible links. all 3 tested ok, that's not to say i am not going to pick up an extra for the trail.

I did not try to modify the VAF, which i don't even know what is.
an interesting thing i did notice was that the fuse has been removed from the ECU slot in the fuse box. but the vehicle has been running this was, is there a chance somebody did some modifications in this area or is this normal?

Thank you for your help, i am going to try to follow the instructions on your diagnostic test.
 
I did not try to modify the VAF, which i don't even know what is.

If you don't know what the VAF is, how can you be certain that you didn't "modify it"? The problem is not someone modifying but trying to disconnect the harness by unscrewing the connector.

You originally stated an intermittent starting problem then testing the fusible links. Depending how rigorous your test was, it may have not detected a problem. While resting you need to wiggle each link as you have a meter connected.
 
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The air flow meter (AFM , not VAF or MAF) is the small box attached to the air filter housing with the forward facing flat connector. Many people mistakenly remove the 2 Phillips screws on the side of the connector housing as opposed to lifting the thin spring clip to remove the connector. When they do this and pull the connector out of the housing, the small internal wires get torn off their solder joints and the AFM no longer functions.
However, this is typically not an intermittent condition, but more of a permanent no-start/no CEL.

Fusible links can go intermittent as they age. I usually replace mine with every battery change.

That being said, it can be frustrating trying to solve an intermittent problem. Rather than throwing rocks at things, I prefer real diagnostics.

The missing fuse: I assume you're referring to the ECU-B fuse in the top right of the fuse panel by the driver's left knee. There was never a fuse there and if you look closely there are no fuse blade terminals in that location.
 
However, this is typically not an intermittent condition, but more of a permanent no-start/no CEL.

This was not the case with mine, it was an intermittent fault. That was my experience, but I agree with the above statement as a generality.

To the OP, did you get it sorted?
 
I'm waiting on the fusible link to arrive.
I've been going through the system diagnostic that jonheld referenced in post #3. much thanks jon i've learned much.

so the interior lights work, the charge light comes on at start, and brake lights work
i get +12v @ efi relay
+12v @terminal screw to fuse box
low v @ pin B+

if i push down on or wiggle the relays in the fuse box under the hood then the charge light will go out at key in on position
occasionally the CEL will come on from doing this and the vehicle will start. it is currently dead in the driveway

i am thinking the wiring leading into the bottom of the fuse box is causing issues?

again thanks for the help
 
fusible link installed. the problems still exist.

i had my buddy sit in the drivers seat while i wiggled wire harnesses and connections. when i wiggled the fuse box under the hood the CEL came on and the vehicle started. also I checked the B+ pin and was getting +12v where i was only getting low voltage.

should i remove the fuse box and look below for damaged wires or connections?
 
Had a similar problem with my 94. Bad connection at efi relay or efi fuse. Two things I would check at the engine fuse box that holds the relay. Make sure the spade connectors are tight. The female connector that the relay connects to. In the same fuse Box there should be a fuse that feeds the relay. Make sure that those pins are also tight and there is a good connection.
 
vkasm1 you were right it was a bad connection from the wire connector to the spade of the EFI relay, I tightened the connector and it resolved the issue. During this the same began to happen to the charge lamp relay.. I did the same procedure to tighten the connections however i cannot get my charge lamp to come on when i turn the key to the on position. the vehicle starts but what am i risking by not having this charge lamp relay work correctly? i'm getting 14.5v at the battery while the vehicle is running
 
That relay seems to control the lamp. I would think if your alternator does fail while driving you won't get a lamp illuminat on cluster. I would check the 2 fuses ignition and charge and possibly another relay to see if you get the lamp on.
 
fusible link installed. the problems still exist.

i had my buddy sit in the drivers seat while i wiggled wire harnesses and connections. when i wiggled the fuse box under the hood the CEL came on and the vehicle started. also I checked the B+ pin and was getting +12v where i was only getting low voltage.

should i remove the fuse box and look below for damaged wires or connections?
I had similar issues w/ my (wife's) rig: EFI relay connections loose. Jonheld is the expert that has, and continues to post soooo much info to help us noobs with this type of problem... Glad you got it sorted out.
 

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