Darth_V8r

iTrader: (4)

Ok, I'm only posting this after searching and not finding similar combinations of symptoms and after doing the diagnostic work that I can do on my own. I am extremely confident it is not in the tune, but there is a pattern to this.

The car runs fine, idles great, rarely if ever stumbles. Zero complaints with day to day driving.

While driving along, I will get a random hanging idle, where it won't drop below 1500 until I come to a dead stop. The car will not stall or anything. However, the very next time I go to start the car, it will not start unless I press the gas pedal while turning the key. Then, it fires up, flares, settles down, and runs fine again for a week or two. No codes at all. Does it with MAF or without

SO, my thoughts are:

1. TPS failing
2. IAC resetting randomly
3. Fuel pump failing

thoughts?

wssix99

iTrader: (5)

How is your fuel pressure and have you changed the fuel filter lately? I'd look there before the pump.

Darth_V8r

iTrader: (4)

Filter is a year and a half old. Pressure bounces 53-58 idling. I don't know what it reads at WOT, but my AFR on my last dyno run was 12.5-13.0 through all the pulls and never leaned out.

wssix99

iTrader: (5)

Odd that pressing the accelerator solves the problem. ... That's very carburetor...

If you have the fuel pressure, then that would indicate that your pump is good and the act of pressing the pedal isn't really about fuel flow.

It might be interesting to watch the pressure gauge during start up to see what happens and if it drops.

Darth_V8r

iTrader: (4)

Lol on the carburetor. That's why I'm wondering if either the TPS or IAC are somehow randomly resetting, giving high idle counts (hanging the idle) and then somehow preventing the next start. Pressing gas simply gives some air not coming from IAC?

On the FP, my thought was electrical. But I couldn't make pressing the gas make sense either. I'll have to test fuel press when I have someone available to start and watch the gauge.

wssix99

iTrader: (5)

It's been a while since I messed with the TPS, but its a pretty basic sensor. Just a varistor hooked up to the throttle plate. I recall it can be bypassed by unplugging the sensor and jumping some pins. (To simulate the throttle closed situation.)

I like your IAC idea. Interesting.

hrcslam

It could be a sticking IAC valve. Try removing and cleaning it and the TB.

The other thing it could be is a leaking injector. IIRC, holding the TB open at start up tells the injectors not to fire (but only WOT).

Both of these can cause an intermittent high idle. I'd lean towards the IAC if you're getting no fuel pressure bleed down after prime.

Remember that your engine has 2 idle targets, one at 0% TPS with speeds over Xmph (defined in the tune), the other at a stop (a stop is also defined in the tune, less than X pulses per minute).

Also, it's worth noting that a datalog doesn't show actual IAC position, only it's commanded position. So the PCM may show 55 steps IAC, but that doesn't mean the IAC is actually at 55. If it's sticking, it could be somewhere else. But the computer won't know that, it'll just command more or less steps to aquire the target RPM. This is to say, if you datalog it and notice the IAC steps change, but not idle you know where to look.

The IAC and TPS also have a self relearn procedure, but I forget what exactly makes it relearn. But that maybe what makes the condition intermittent if it is the IAC.

Darth_V8r

iTrader: (4)

Thanks, when I get home tonight, I'll pull the throttle. It is brand new (well a month old), but possibly a quick wipe with some acetone to clean up any residue is worth trying. The IAC and TPS are both still original. Tuner suggested possibly something was blocking the blade from closing all the way, so I can just clean the whole thing really good.

After that, I'll run a start up scan and look to see if the IAC is moving more than the idle speed.

The relearn procedure is to disconnect the TPS, key on (no start) for 30 seconds, key off, plug in TPS, start car. Done it a million times, but, as I said, car idles great. Only hangs right before it's about to give me a no-start.

Darth_V8r

iTrader: (4)

Throttle body is squeaky clean. If it happens again, we will know that wasn't it.

Darth_V8r

iTrader: (4)

Did it again today. This time I noticed on letting the fuel pump run (key on engine off) it took much longer than normal to stop running. Then fired up.

I'm thinking either the fuel pump is going or it is not getting good power during start.

Bench tested battery and is ok.

I guess I'm going to be getting a Racetronix and Hotwire kit. I still want to pressure check to be sure

hrcslam

Can you clarify what you mean by letting the fuel pump run? When you turn it off or before you start it? What do you mean took longer to stop running? The fuel pump took longer to stop running?

I think I'm understanding that if you let the car prime longer before you start it, it starts up better?

If so, I'd definitely do a fuel pressure check. One trick you can do is prime the engine 3 times before you attempt to start it. That points directly to a fuel issue. If you have a returnless fuel system, it's either the pump or a leaking injector. If it's a return fuel system it could also be the FPR.

Darth_V8r

iTrader: (4)

After the no start, I keyed on to prime the pump. Instead of 3 seconds, it ran 5 or 6 seconds before cutting off. Not sure if that is related to pressure feedback or a timer ran long, but it is not normal behavior.

Yes, that's correct.

I did the pressure check and pressure fluctuates 53-58, but doesn't dip below 53. I also need to try to check pressure while the starter is running, to see if I'm losing voltage, as it never cuts out once started.

It did start better after multiple primes. Instead of cranking and cranking with the pedal down, it started with no pedal, but it sort of stumbled to life, if that makes sense.

It is a returnless system.

I guess it could be a leaking injector, but when I run with HPtuners scanning, I'm not seeing anything in the fuel trims or the O2 sensor voltages that is making me think there is an issue. Is there something I can do instead to verify injectors? Would a simple fuel pressure leak down test show this?

hrcslam

After the system primes, check the fuel pressure guage, if it drops fairly quickly then there's a leak in the system. If not, no leaks, but if it takes a while to prime up to pressure the fuel pump motor is going out.

Darth_V8r

iTrader: (4)

Ok, so here's where I am on this. I had a friend help me and ran the following tests with results:

1. On start and prime, spikes to 58-60 psi, then drops to 51 after the three seconds are up and holds 51 for a good couple of minutes without falling off

2. Start her up, and holds 53-ish at idle. Revving, pressure will dip to 49/50, then recover to 60 and settle back in at 55.

3. Rear lifted, using brakes for load, runs 49/50 under load without getting stupid. Only way I could tell what the pressure is doing at high duty cycle, but I never broke 20% doing this anyway.

I can't think of anything else to try, but I'm guessing this all means the issue with the no starts is electrical, so I'll be ordering a Hotwire kit, unless someone tells me what else could be at fault.

hrcslam

Yeah, your fuel pressure is good on all fronts. No indications of any fuel leaks any where.

Time to look else where.

Darth_V8r

iTrader: (4)

Thanks HRC! Saved me a project!

hrcslam

Honestly, my LT1 has done this since my rebuild. I don't pay attention to it any more. There's no reason for me to try and fix it other than it's a little annoying. The high idle while rolling is kind of nice sometimes. The hard start is no biggy as I just pedal the throttle a bit and it's running. Give it a minute (usually less than) and it settles and all is good again. I attributed it to my mods. Nature if the beast.

Darth_V8r

iTrader: (4)

Sorry to bump but better than a full new thread for a simple question...

My last WOT run I was at 70% injector duty cycle on 40# injectors with AFR at 12.8. My pump is still stock. Anything I can do to monitor fuel pressure under these conditions?

hrcslam

Yes. Put the fuel pressure regulator on the window in a place you can see it while driving.

Darth_V8r

iTrader: (4)

Just in case this comes up in a search later, I fixed it in the tune. There is an air adder during start in HP Tuners. I found a reference to it on a thread about difficulty starting larger cams when there is no apparent issue. I edited that table and it hasn't happened since. It does flare to 1500 on start but I don't care. It's better than wondering what's wrong.

I'm guessing here - I think the added air at start up is forcing the IAC to open more, which is guaranteeing enough air is getting in. Hence the reason using some gas pedal corrected the no starts before.