Fuel Continuous FP Prime - Codes 87 & 95

BobV

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May 18, 2003
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Working on the 1989 LX 5.0 /AOD higher mileage car.
Runs good, except fuel pump does not shut off at pre-start prime (KOEO) and car "hiccups" repeatedly above 4000rpm at WOT.
Plugs, wires, cap, rotor and fuel filter all done a few months ago. Fuel pump relay replaced - no change.

Pulling codes gave 87 & 95, both relating to fuel pump circuitry. In reading up (yes, I searched enough to get a headache), most with those codes have a no-start condition. This car starts and runs fine (been DD'ing it). Other than the 4k+ hiccups and the time I ran it out of fuel a few weeks ago, no issues, so I don't think I'm really looking for a wiring issue. I have NOT yet checked fuel pressure, but it's on my list.

Is my next step pulling the ECM to check capacitors/board burns, or is there something else I should be checking? Thanks in advance.

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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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Fuel pump regulator and fuel pressure is next. Have you checked out the 'surging idle checklist'? It goes over a lot of things that can involve 'hic-ups. It's in the 'technical/how to' thread.
 
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Mustang5L5

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Continuous fuel pump run is often a sign there is an ECU issue. I would go ahead and pull the ecu and inspect the capacitors and test for continuity between pin 46 and 40/60.
 
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2000xp8

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I've seen the relay get stuck.
You could try replacing it or swapping it out.
Or whack it with the back end of a screw driver.

No idea if it still triggers a code. Back in the day i don't think anyone i knew actually looked at codes.
 
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Mustang5L5

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Mod Dude
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Good suggestion on stuck relay. It’s worth checking.


I’m not convinced the code is tied to this issue. It might be an old code and one worth erasing and verifying if it comes back.

Pin 19 on the ECU monitors the fuel pump circuit after the relay. If there’s no power on the wire that runs the pump, pin 19 won’t see voltage and will trip that code that says the fuel pump isn’t running.

In this case the fuel pump IS running. So under normal conditions that code shouldn’t appear. That suggests to me it’s an old code that just needs to be erased, or the circuit for pin 19 and the fuel pump circuit is compromised. However…the pump runs.

I guess the question to ask is OP, are you aware of any wiring changes or hackery to the fuel pump circuit (or any circuit) on this car?
 

BobV

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Good suggestion on stuck relay. It’s worth checking.

....

I guess the question to ask is OP, are you aware of any wiring changes or hackery to the fuel pump circuit (or any circuit) on this car?
I changed the relay when first attempting to diagnose the FP prime issue a month or so ago. Made no difference.
Not seeing any hacked wiring (other than the stereo which I have now cleaned up).

Had a busy weekend, so I did not get to dig into this any further. The car runs fine other than the prime & hiccups issues. I'm re-upholstering seats this week, so I'll try to pull the kick panel / ECU and just have a look to see if anything OBVIOUS sticks out inside.

After that, I'll clear the codes and drive it for a few days, check fuel pressure / regulator, and re-check codes. At least that's the plan...
 
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Mustang5L5

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I’ve actually been digging around inside the A9L a bit in an attempt to study it.

I believe one way to test to see if this issue is the ECU is to perform a continuity test from pin 22 to pin 40 or 60 with the ECU disconnected. With a good ECU, you won’t have it, but on a bad one I believe you will have continuity.

The fuel pump is driven internally by a transistor with a few resistors. A common way this setup blows is by wrongly applying 12v to the fuel pump test circuit on the sto test plug, or wrongly applying 12v to the fuel pump relay on the wrong side of the coil trying to manual activate it.

Either the resistor blows, or the transistor shorts. If the fuel pump doesn’t run, it’s the resistor. If it runs constantly, it’s usually the transistor that needs to be replaced.

I don’t have a bad ECU to test my test methods and repair on, but I’m fairly confident that short of any wiring issues, this is an ECU issue, specifically the fuel pump transistor.

If you get a chance, if you remove the ECU can you test continuity from pin 22 to pin 40 or 60?
 
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2000xp8

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Honestly, if you are going to crack it open, imo just repair those parts.
Some things can be tested in circuit some can't.
You could most likely test the transistor mike listed as the issue by just checking continuity at the legs, but as i said i've seen times on electronics that works and times it doesn't. So if you have to remove it you can test it vs a new one.
Me personally, i'd just do the caps and transistor while its apart.
That's assuming you are good at desoldering (soldering is kinda easy, desoldering, not so much). Personally i own a Hakko FR300 so i don't really shy away from the task like i used to.
 
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Mustang5L5

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Mod Dude
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I agree. Testing some of these parts is tricky to do on an assembled board. If you can do soldering, this is a $10 repair to change out that transistor and replace the 3 caps.

The fuel pump transistor is this one I circled in red. It's pretty easy to identify which one as the center post of the transistor rings out when you test continuity from pin 22 to it. I think a MJF3055G transistor would be an acceptable substitute. The three capacitors are also an easy change out.

Of course i realize not everyone is comfortable doing this sort of thing

AC5E4DC9-740A-452F-BD30-C2D62C538201.jpeg
 
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BobV

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May 18, 2003
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OK, finally got some tinker-time.
We have continuity between pin 46 and 40/60.
No continuity between 22 and 40/60. Read ~470 ohms resistance. I do have continuity between 22 and the center leg of the circled transistor.

In your picture above, the upper/top capacitor does have some green residue at the base.
The capacitor on the far right appears to have a broken leg. Not sure if that's new damage from trying to get a good look, or pre-existing. If it IS new, it was definitely weak anyway.

I have all 3 capacitors and a transistor here. I'm definitely changing all 3 capacitors. May plug it back in at that point just to test - out of curiosity. Or, I may just get on a roll and do the transistor while i am there. Probably won't be tonight, but soon. Will update again............
 

Mustang5L5

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Mod Dude
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I would do the capacitors first and test. I wouldn’t touch the transistor at this point.

I believe the far right cap is related to fuel pump function and believe repairing at least that one (although do all three) may solve the pump issue.
 

BobV

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Welllllll.....
The 3 capacitors did not resolve the issue.
Next was changing the transistor, and that did not resolve it either.
Same continuous prime issue, same 87 & 95 codes. :eek:

At this point, I'm going to drive & enjoy it for a few more weeks until the weather sucks.
I've got a few other ideas too.
 

Mustang5L5

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Mod Dude
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Code 85 and 97 don’t make sense here. Those codes pertain to the pump not running when they clearly are. Generally the lack of power on pin 19 will trip this code. With the pump running this should not be the case.

Are you absolutely sure the fuel pump relay wiring has not been tampered with? Have you pulled back the carpet to check? Any chance a PO installed a fuel pump kill switch.

With the car running, can you check for voltage on pin 19 to ground?