RB25 starts fine, then idle hunts and dies after it warms up.

[optimusprime8]

Hello!

I am stuck w/ my RB25 swapped 240z. Here is the setup and behavior:

Basically stock RB25DET in a 240z chassis with Wiring Specialties harness, fake Greddy intake manifold, front mount intercooler, and manual boost controller. Everything else is stock, I think.

The problem is when I start the car, it runs perfectly for about 5 minutes, but once it is warmed up, it starts to idle hunt until it eventually dies, the car will not start again until it has cooled down.

I have replaced with brand new:
Idle air control valve/solenoid / AAC
Coils
CAS

There are no codes being generated, I'm using ECUTalk to look at the output of the stock computer. When the issue occurs AAC is at 100%. I also put a volt meter on the two wires going to the Idle air valve, one is constant 12v, the other starts around 10v when I start the car, and drops to ~1.5v when the issue occurs.

I am stumped, I did test the output of the temp sensor and I think it is good too, but maybe I should replace it as well. Any troubleshooting tips or advice is appreciated, I'm about to throw in the towel and take it to a pro.

[VStar650CL]

Does the exhaust start to smell rich as the car warms up? What do your plugs look like? If they're black and sooty, definitely look for either a bad temperature sensor (IAT or ECT) or a bad fuel pressure regulator. The diaphragm on the latter can leak gas back into the intake if it's blown, which the engine will tolerate when cold but causes it to drown once it warms up.

[optimusprime8]

Yes, it is absolutely dumping fuel, it is rich for sure. I will try a new FPR. Thank you!

[VStar650CL]

You're most welcome. Post back what you find. The FPR is easy to check, run the engine for a short time and then pull the vacuum hose off it. If fuel drips out, you found your culprit.

[optimusprime8]

I replaced the fuel pressure regulator with one from an Infinity Q45, I believe it is the same. No change, I still have the same issue. I did confirm there is fuel in the return line. I think I will order a fuel pressure gauge next, it's cheap. It still smells rich.

I also replaced the ECU temp sender, no change with that either.

I was thinking about trying a MAF and maybe a VTC solenoid too, since I'm just throwing parts at it at this point.

I did confirm continuity on the wiring between the Idle Air solenoid and the ECU. I suppose I should do that for the temp sender as well, maybe it's a wiring issue.

[optimusprime8]

New MAF didn't help either. Fuel pressure at the gauge is at ~40 plus or minus 2. I'm at altitude so possibly that affects the gauge reading.

It's still rich as I can smell fuel and see liquid fuel coming out of the downpipe. I don't have an exhaust on it yet.

[VStar650CL]

Did you ever yank the plugs to see if particular cylinder(s) are running rich? The next suspect on the list would be an injector stuck open.

[robotbbq]

I had the injectors cleaned and tested at RC engineering, but then the car sat for ~10 years before I got around to working on it. It's possible one of them is stuck.

I pulled the spark plugs and did the test where I turn the CAS unit manually while looking for sparks and hearing the injectors click. All the injectors are clicking, all the sparks are sparking, but I suppose an injector could be puking fuel even if it is clicking too.

I'll look into testing injectors further or maybe get a new set. Thanks.

[VStar650CL]

if it's a leaky injector than there should be evidence, that particular plug and cylinder should be covered in soot and markedly darker than the others. If they're all sooty then it has to be something global. Does it rev normally when cold, have you excluded breathing issues like a blocked exhaust?

[AZhitman]

Fuel pressure seems a tad high as well - fuel pressure at idle with the vacuum line connected : 32–36 PSI, vacuum line disconnected: 43 PSI

My turbo KA did the same for months, but mine was an intermittent bad IACV.

[optimusprime8]

I put new 370cc injectors in it. It is not dumping fuel now and isn't as rich, but still dies as soon as it is warmed up.

I am thinking it might be a water pump or thermostat issue. I don't think the thermostat is ever opening, the top rad hose is dry inside. I haven't taken the bottom hose off yet. The thermostat is hard to get to with the greddy intake.

I replaced the thermostat, water pump, and timing belt about 10 years ago, but the car sat for a long time with no coolant in it, in a garage, but still...

I am speculating that the ECU temp sensor never gets hot enough, because there is no coolant flowing, so the ECU doesn't have the data it needs to deliver the correct amount of fuel based on engine temp.

I tried burping the coolant system by jacking up the front of the car and running w/ a big funnel in the open radiator, it maybe took a quart, not much fluid at all. I still feel like I am not getting any coolant in that top hose, so I'm thinking:
1. Pull thermostat out and test it in a pot of boiling water to see if it opens.
2. Verify water line routing
3. Replace water pump if above are good.

I'm not really wanting to do the water pump/timing belt again, it's a lot of work, but I suppose sitting for a long time could seize up a brand new water pump.

[AZhitman]

Yep. Also, pull the water temp sensor. If the t-stat is gunked, that is too.

No coolant means no anti-corrosive which means you may have a cooling system full of rust.

[optimusprime8]

I found another issue, I hadn't noticed this before, but I don't know if I missed it or if it is new.

ECUtalk shows the Throttle V value at 5.10. But when I measure the voltage at pin 38 on the back of the ECU, at closed throttle, it measures at .44, which I believe is correct. It goes up to 4.00v when I go to wide open throttle.

Maybe my ECU is fried on the TPS circuit? TPS was one of the things I replaced in this process. Maybe my pinout for my ECU isn't correct for the ECU?

Part number on the ECU is 23710 21U60

[AZhitman]

That's a R33 manual ECU. I'm not really sure how you'd test it, though.

That reading would imply WOT, not closed-throttle idle. Something's wonky for sure.

[optimusprime8]

Progress! The issue with the TPS was that the pin on the ECU wasn't making contact with the connector properly. I jiggled it, and messed with the connector tightness and now the TPS is reporting correct values.

It will now run longer than I have had it run at any point. I got it up to 190 degrees before the idle hunting and stall. It will restart when hot now too, but then idle hunts and dies. I've unscrewed the idle control on the AAC/IAC valve quite a lot, that seems to help with the hunting.

I think I need to set the timing because I replaced the CAS. I am also getting knock sensor alarms. The knock sensors and the VVT solenoid are now the only sensor-type things that have not been replaced with new items at this point. The knock sensor connectors are loose and come off easily. I suspect the sensors might be good, but the connections aren't consistent. I am planning to zip tie them on and see if that improves anything.

ECUTalk appears to be reporting valid values for all sensors, but it doesn't have values for knock sensors.

So next work will be:
1. Set timing
2. Set idle
3. Zip-tie knock sensor connectors.

Is there any other setup that needs to be done after replacing pretty much every sensor?

[AZhitman]

Damn - Nice sleuthing!

I've never had good luck with an IACV after it's been adjusted, but trying to find where it *should* be set can be a problem.

KS are finicky. Timing, cold idle should be all of it, but I think I'd figure out if you can get good KS readings before all that.

[flartius]

It's still rich as I can smell fuel and see liquid fuel coming out of the downpipe. I don't have an exhaust on it yet.

Do you think it could be getting too much oxygen to the 02 sensor? Once it warms up and gets out of the warmup loop and starts to actually read the 02 sensor reading and adjusting the fuel map? If it gets too much oxygen at the sensor it will keep increasing the fuel trim to try and burn up all the oxygen in the exhaust.. VStar probably knows more about this than I but its one of the few remaining pieces that I see.

[VStar650CL]

Do you think it could be getting too much oxygen to the 02 sensor? Once it warms up and gets out of the warmup loop and starts to actually read the 02 sensor reading and adjusting the fuel map? If it gets too much oxygen at the sensor it will keep increasing the fuel trim to try and burn up all the oxygen in the exhaust.. VStar probably knows more about this than I but its one of the few remaining pieces that I see.

It would be pretty unusual for a front O2 to "lie" to the ECM without throwing codes, but a pinhole leak in the exhaust near the sensor could fool it and have that effect. I'm not sure that the open downpipe couldn't cause that all by itself, but pinholes are easy to find just by blocking the exhaust with a shop rag. That will make even a tiny hole into a tiny jet engine which will be very obvious to a "feel" test.