The car has performed flawlessly until last week


2 min into a drive, something triggered a problem, which oddly enough blew the gauges fuse.

replaced it - pulled a 33 code, then a 34 code
maf, maf circuit/high vaccuum etc.


so far, tried a new ECU, then a new maf relay by the brake booster, replaced the maf as well.

every time, car fires up great, sounds good for 15-30 sec, then goes into this mode of coughing black smoke, and erratic idle

yesterday while running, i unplugged the maf burnoff relay, and you can hear the contact triggering on/off when you unplug it but it didnt seem to help, I also unplugged the MAF itself, and when plugging it back in, the engine came back to a normal running state for 10-15 seconds before going back into this low idle/black smoke.

i have a replacement bosch burnoff relay coming - but i don't think thats the problem, as when i was digging into that, i could manually flip the relay inside - not sure what it was supposed to do but it had no impact on the way it ran.

what next? Any ideas?
no vac leaks anywhere

 

What does the fsm troubleshooting tree say to do next ? Replace maf ?

 

already done that

 

Ecm seeing how many gps at idle ? Connection seems like it could be a problem?
tpi parts.net has a pigtail for the maf.
btw oddly enough I’ve seen misadjusted tps cause maf codes.

 

yes possibly - gonna go get a scan tool today so i can see what the ecm is logging


odd that just out of knowhere, it started behaving like this.

 

A random thought here;

The gauges fuse, which feeds the pink/black wire going into the gauge cluster, also feeds the pink/black wires powering some of the underhood engine controls. If I recall correctly the EGR solenoid and the air select valve are two of the devices, and I think there may be more. Perhaps inspecting the wiring and devices under the hood associated with the pink/black wires may turn up something?

Given what I know of the gauge fuse being involved with more than just the gauges, I don't think it's a pure coincidence that the gauge fuse blew, I think it's a symptom of whatever went wrong.......

 

This is a 1986 TPI wiring diagram (sorry I don't have 1985) showing some of the things the gauges fuse powers beyond just the gauges. Look at the lower middle part of the diagram to see some of the things the gauge fuse powers;

 

The egr solenoid is where my money is

 

thank you!!!
will get on it today

 

Here's the GAUGES fuse wiring schematic from the 85 Shop Manual. It covers all drivelines, so be sure to follow the " VIN F only " on the flowchart.......

** NOTE: As shown,...... The Defroster and the CRUISE Control are also fed voltage thru the GAUGES fuse !

 

Just found the 85 TPI MAF schematic and wanted to add it here for future reference !

 

OK - OK - I know I'm outta' control !!!!!

Here's TPI Cold Start and all the engine bay stuff related to the GAUGES fuse so you can see the wiring from the valves/relays/sensors back to the ECM.

 

Update - still random-

i thought we had it - It ran and idled beautifully for an hour yesterday, 2 test drives etc.. not problem…

then last night, boom, gauge fuse blows again, all goes to hell.

so far we’ve tried a different ECU, new MAF, new MAF relay - still waiting on a known good MAF burn off relay - Can’t get a new EGR solenoid so can’t replace it -

planning to pull the plenum today to check the EGR, but i dono’t think a stuck EGR has any impact on how the EGR sol works.

when its not misbehaving, it runs great, then out of no where it seems to go into an overly rich mode, backfires, runs like **** - and gauge fuse blows simultaneously.

any further thoughts? I’m gonna try the new ECU again, as both times the gauge fuse blew, it was with the OG ECU, but changing the ECU seemingly made no difference…

hard to believe two things **** the bed at the same time….

 

Get yourself a handful of good 10 AMP fuses

Unplug each "thing" that's connected to the GAUGES circuit 1-at-a-time and run the car. ( example: cruise control )

If the gauges FUSE blows when the Cruise is disconnected - it's NOT the Cruise blowing the circuit. Plug it back in and unplug something else ( example: rear defogger )

If the gauges FUSE blows when the Rear Defogger is disconnected - it's NOT the Rear Defogger blowing the circuit. Plug it back in and unplug something else ( example: EGR solenoid )

Continue this process as far as you can till you find the "thing" that allows the car to run without blowing the fuse.

EDIT ** These test only check the "thing" and not the wiring to that "thing" !

Always inspect all the plugs wiring close and worry about voltmeter testing ( if needed ) after you've eliminated as many "things" as possible. Obviously you can't check the ECM in this way, but at least you can reduce the list of possible suspect components & help narrow down the culprit rather than just guessing which of the dozen or so "things" could be causing the fuse to blow. Heck; you could have a bad printed circuit on the back of the cluster, or a chaffed wire at the cruise connector,..... You gotta' start somewhere and make real progress rather than just guesswork.