20V Blacktop Swap - Suddenly Dying at Idle after Cold Start

309 Views 16 Replies 3 Participants Last post by mr220v, 9 d ago

discorporation

Discussion Starter · 1 mo ago

Reaching out to see if anyone might have some insight on an issue I've been troubleshooting. Car was driving fine until about 2 weeks ago and then this randomly started happening one morning.

20V Blacktop Swap on 87 Chassis - Been in the car for 7 years now and has ran fine otherwise, ~10k miles since being swapped in with major refresh done at that time. Engine is stock with factory airbox on it.

Runs fine from a cold start for about 1 minute then starts to exhibit this behavior. Car isn't storing the CEL so I can't check the code (leads me to believe its actually dying when the CEL turns on instead of throwing a code for something). I've drained the fuel tank and refilled with 5gal of gas, replaced fuel filter and ran fuel gauge inline with rail (~38psi). Jumpered Fp +B to force the pump to run continuously, the fuel pressure does bounce around a bit when it starts to stumble but never falls below 25psi and recovers as soon as the engine does. Forced ECU into open loop by unplugging Coolant Temp/TPS/O2 sensors individually and all at once, verified TPS and Coolant sensor values were in spec per BGB(Blacktop one on SQ Engi). Unplugged MAP but will barely run at all unless you feed it some throttle, there is a vacuum signal present at MAP. Replaced Igniter/Coil with known good ones off of my SC400 (blacktop ones seem to work fine on Lexus). Inline spark tester on each cylinder, the spark looks fine until it starts to exhibit these symptoms then signal becomes erratic and intensity increases/decreases randomly but like the fuel pressure as it recovers briefly the spark steadies. Removed and cleaned the IACV, ohmed out correctly. Performed a smoke test and replaced a few vacuum lines, no leaks now. Ran with and without VVT solenoid. Pulled ECU and checked caps (these were replaced with new at time of swap). If I hold the throttle steady at 2000rpm it will continue to run but the RPM still dips about 250-500rpm randomly, CEL comes on briefly and then it recovers so similar behavior as at idle just higher in the rev range. It is for the most part drivable like this but you have to hold the throttle open when coming to a stop or it will die and then it has difficulty restarting.

Any insight is greatly appreciated.

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rmeller

· #2 · 1 mo ago

I am thinking an interruption in power distribution. The fact that both the fuel pump and ignition are stumbling while the engine is still running suggests a common cause.

discorporation

Discussion Starter · #3 · 1 mo ago

rmeller said:
I am thinking an interruption in power distribution. The fact that both the fuel pump and ignition are stumbling while the engine is still running suggests a common cause.

So a wiring issue in your opinion? I checked the engine bay fuses, would a failing relay be a potential culprit? COR was within resistance specs per BGB.

Any suggestions on where to start looking for issues? Literally nothing was done to the car, between driving it and this behavior other than sitting a few weeks. I also installed a new battery when I began troubleshooting. I purchased a new MAP sensor since I didn't have a means of testing it and changed it out today without resolution. Pulled the timing covers and verified mechanical timing as well.

Thanks

discorporation

Discussion Starter · #4 · 1 mo ago

Just messed with it a bit more forcing it to run at 1500rpm, CEL on solid and then it abruptly shut off but there was no CEL light with the ign still on and now I do not get a CEL when I turn to ACC with the key and while in ACC the IACV is clicking nonstop. INJ and EFI relays both feel quite warm, almost hot and the COR relay is a bit warm as well but not as bad as the other two. When starting it will just immediately die unless I give it some gas and is running noticeably poorer with the throttle open. When I try to jumper TE1 and E1 I get no CEL either. INJ and EFI relays do not have continuity between 1/3 per BGB(70 ohms across, age?) but the coil does energize when 12V is applied and 2/4 have continuity when coil is energized.

It will still rev out but is cutting up pretty bad now. Fuel pressure still seems OKish but hard to check the gauge since I have to be on the throttle when starting now. I don't really want to drop the tank but is there any reason to suspect the fuel pump is dying? Was replaced with a denso unit at time of swap.

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rmeller

· #5 · 1 mo ago

discorporation said:
... there was no CEL light with the ign still on...

... now I do not get a CEL when I turn to ACC with the key and while in ACC the IACV is clicking nonstop...

... is there any reason to suspect the fuel pump is dying?

This suggests a global electrical distribution problem.

You mean RUN, not ACC, right? ACC is accessory, and nothing is supposed to be running in accessory. If this is really happening as you say, then this is a real problem, so make sure that you are clear on this.

Don't clutch at straws. Your ignition switch may be malfunctioning, particularly if you are getting the wrong circuits energized as you suggested above. Another possibility is the main fusible link. This is a heavy duty fuse that is spliced into the main power feed and connects it to the battery positive. That can corrode and become intermittent.

discorporation

Discussion Starter · #6 · 1 mo ago

rmeller said:
This suggests a global electrical distribution problem.

You mean RUN, not ACC, right? ACC is accessory, and nothing is supposed to be running in accessory. If this is really happening as you say, then this is a real problem, so make sure that you are clear on this.

Don't clutch at straws. Your ignition switch may be malfunctioning, particularly if you are getting the wrong circuits energized as you suggested above. Another possibility is the main fusible link. This is a heavy duty fuse that is spliced into the main power feed and connects it to the battery positive. That can corrode and become intermittent.

Yes sorry, RUN, the position prior to engaging the starter. I ordered a set of thin needle multimeter leads so I can better measure voltage at the ECU pins. I find it strange that the IACV just clicks non-stop indicating some kind of wiring issue thats causing it to receive a signal(s) while the engine isn't running unless its normal behavior...thinking back it has always clicked a few times prior to starting and right after shutting the car off, but never continuously while the engine isn't running.

discorporation

Discussion Starter · #7 · 1 mo ago

2

Just some additional observations, brand new BK6RE-11 plugs with 0 miles just idle/troubleshoot time yesterday. All 4 look the same.

Automotive tire Finger Bottle Nail Automotive design

Automotive tire White Road surface Black Asphalt

Automotive tire Finger Bottle Nail Automotive design

Automotive tire White Road surface Black Asphalt

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rmeller

· #8 · 1 mo ago

Check the main fusible link. If that is clearly solid, then work your way on to the circuit that powers the fuel pump and ECU. Did you make any connections for the swap? If so, check them first.

discorporation

Discussion Starter · #9 · 1 mo ago

rmeller said:
Check the main fusible link. If that is clearly solid, then work your way on to the circuit that powers the fuel pump and ECU. Did you make any connections for the swap? If so, check them first.

I pulled the sheathing off of it and tested continuity from the ring terminal to the end of the silver wire and from the terminal to the thicker gauge copper wire then from the ring terminal to AM2 and all were good. Wiggled the wire around to see if I could get it to interrupt but seems solid. Harness was done by WireGap, only addition I made was a relay to ground the AC Idle/Up pin when switching the AC on (got the idea from an old post from AlastairMR2), but again it has always worked up until now so I don't necessarily see that addition causing an issue. Doug actually responded to my FB post on MR2OC suggesting its an ECU issue, I could see that possibly being a culprit since the only thing that would send a signal to the ISCV would be the ECU (or a short somewhere) since it was going nuts energizing the solenoid non-stop the other evening after shutting down.

rmeller

· #10 · 1 mo ago (Edited)

discorporation said:
...suggesting its an ECU issue..

... the only thing that would send a signal to the ISCV would be the ECU ...

Maybe, if the ECU controls the fuel pump. In the mk1, the fuel pump is stand alone, but your car is probably different.

The ECU may send a test sequence to the idle valve when it boots up. I know that some cars do that. The idle valve thrashes back and forth a few times to verify that it works. This sounds like the ECU repeatedly trying to reboot, and that could be due to bad power. Look into the ignition switch next. Try jiggling it to see if you can provoke this result.

discorporation

Discussion Starter · #11 · 18 d ago

Just figured I'd follow up on this, I purchased a used blacktop ECU on eBay and have been driving the car for the past few days as this seemingly was the cause of my issue. I installed the new ECU and left the car to idle for around 20minutes without seemingly any issues. I jumpered TE E1 and checked timing to ensure I was a 10deg BTDC unjumpered the diag port and drove it around the neighborhood a couple laps. Had no issues and once I got back home I put the old ECU back in the car, it lasted about 1 minute before it started exhibiting the same symptoms of stalling out/running rough. Put the new one back in and then let it sit at idle for about 10 minutes without issue before deciding it was fixed.

Now if I only knew how to diagnose the ECU to determine the failure if I ever encounter issues like this in the future. The capacitors showed minor signs of leakage/swelling but there was no damage to the board or traces. Replacing them made no difference where-as doing so in my lexus and vr4 cured the issues they were exhibiting at the time.

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rmeller

· #12 · 18 d ago

Did you replace all of the electrolytics? They can fail without visible evidence.

discorporation

Discussion Starter · #13 · 16 d ago

Yes, all were replaced with matching panasonic units from digi-key. The car behaved the same before and after capacitor replacement so I suspect another component and/or traces (although I did not visibly see any damage) are to blame.

rmeller

· #14 · 16 d ago

You can go after the voltage regulators next, if you have the desire. 90% of all electronic failures are in the power distribution.

discorporation

Discussion Starter · #15 · 15 d ago

I've not heard of replacing those previously, only capacitors, resistors and diodes. Haven't looked in depth at other components on the board before but is it even possible to determine a suitable replacement? I'd love to give it a go but my rudimentary EE skills when it comes to ICs end at capacitor/resistor/diode replacement. I can't imagine they're particularly expensive like most IC components.

rmeller

· #16 · 15 d ago

Regulators are cheap. One reason is that they are as common as fleas on a sheepdog. Most likely any regulator with the same output voltage and footprint will work. Of course, if the device has failed hard, there may be no evidence as to what the output is supposed to be. Most likely, the regulators are there to reduce 12V to 5V, but I didn't design the thing.

mr220v

· #17 · 9 d ago

Not being able to blink codes is your first indicator of a bad ecu. The blacktop ecu's are more reliable than the silvertops, but I guess since they are all over 20 years old, they could get to be a problem as well. One interesting thing I tried with an old customer a while ago was to use and st205 pnp Link with the blacktop harness. There was only one or two wires that had to be repinned. Not the cheapest option, but is something you can buy new vs 25 years old and doesn't require major rewiring.

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