Idle fluctuates periodically even with AAC disconnected.

[mattd1979]

I have had this issue for a while now. The idle wants to kick up and down after sitting idle for a few minutes. It is especially a pain when I am trying to get a power balance test. I have unplugged the IACV awhile back because it would race up really high out of no where as if the car was running cold. I would be sitting at a light and have to hold the brake extra hard to avoid it creeping into somebody. The engine temp is at 174 degrees when fully warm and with the IACV disconnected, I don't know what could cause it to fluctuate at idle. I thought maybe the MAF was having problems and swapped it onto our 1991 Q45 but it did fine.

Matt

[BCC93QT]

Do you have proper fuel pressure?

[elwesso]

I have a similar issue with my car and it turns out there's a funky connection at the ECU harness. Try jiggling the ECU harness while the car is at idle and see if that does anything.

[mattd1979]

OK I will try that tomorrow. Here are the results of my fuel pressure test today for BCC93QT. With the engine off and key on, the fuel pressure was 42PSI. Once started, it dropped to 38PSI. This was after the engine had been off a couple of hours with a temp of 150 degrees. Once warmed up the pressure was still the same. When I gradually increased RPM, the PSI only dropped one pound to 37 PSI and maintained it as I gradually increased throttle up to almost half throttle where I stopped. At WOT the fuel pressure jumped to 46PSI. The engine idle speed according to Scantech was 650RPM with A/C and all lights and accessories off and in transmission in park.

[mattd1979]

Today I swapped MAF sensors between my car and a buddy's Q45 to see if the idle and cruise stumble problem moved to his. I still have the same issues. If the idle air control valve is connected and it starts to increase speed at idle momentarily, the valve tries to bring the idle back down and when the momentary increase in RPM goes away, the RPM drops off even more and the valve then tries to compensate and bring the speed back up which it then over compensates and I have to hold the brake pedal harder to keep it from moving when at a light until the RPM settles back down. If the valve is disconnected, I still notice the RPM jump up around another 100 rpm and then settles back down but it isn't as aggressive. I don't know what can cause the rpm to increase like that when the idle control valve is disconnected. I checked the wiring to the ECU by wiggling the wires to try and provoke problem like Wes said to try but nothing would happen. I'm not sure what else to check at this point. You see my fuel pressure readings in my previous post.

Matt

[Q451990]

When we were trying to get a good power balance test on Nissan Scantech, I seem to remember that the ECU had other tricks to try to stabilize the idle value, besides the AAC valve. Injector pulse width, timing advance, etc. We never did get a test that held every variable constant...

Heath

[goody90q45]

Matt- Do you have a vacuum leak? As I recall you've had the plenum off once or twice so start by checking the 16 bolts that hold the plenum in place. They're easy to strip and can be replaced with a longer bolt and washer with the same threads. I had a similar issue and when I had 3 injectors replaced last month the shop found 4 bolts stripped from previous plenum removals. Bolt replacement also solved transmission hard shifting from first to second gear.

[mattd1979]

What vacuum source would be best to take a vacuum reading from?

[mattd1979]

I figured on pulling the vacuum reading from the brake booster vacuum port.