2004 Suburban Intermittent No-Start (Cranks Normally) and Low Idle

Nicholas McKay · March 3, 2018

I have a 2004 Suburban that has had a low idle (400 rpm) ever since I bought it. I've also had an intermittent no-start (cranks but won't start) that has been getting worse and worse and usually happens in the winter and it has now begun stalling especially when the engine gets warm. It only stalls when I'm parked. Is there anything that can cause a low idle AND an intermittent no-start? I'll explain the symptoms below.

Low idle: If I just tap on the gas, it bumps up to 1000 rpm, then drops off unusually quickly and dips to about 300 rpm before slowly climbing back to 400 rpm, but it never stalls when this happens. If hit the gas and hold it for a few seconds at 2500 rpm or so and let off, it doesn't drop off as fast. The engine slows down at a normal pace until it hits 600 rpm, then slowly decreases to 400 rpm. I'm going to be cleaning the throttle body and MAF sensor today to see if it helps.

No-start: It's not the security system, since it doesn't fire at all. With the security system it will run for 3 seconds, then die. I have also checked the fuses and relay. Seems to be completely random, it probably starts 50% of the time. These "episodes" come in periods of one to two weeks, then goes away for months at a time. I think maybe disconnecting the battery or a dead battery is what usually sets it off. When this happens, the fuel pump doesn't prime and if I jump numbers 30 to 87 of the fuel pump relay, the fuel pump runs constantly and I am able to start and drive no problem. Using a multimeter, I was able to conclude that there is no ground control signal going to the relay from the PCM. The fuel pump draws about 7 amps, which to me, sounds pretty normal. Aftermarket items include 3 amplifiers for 4 subwoofers and 4 door speakers, Blade_Al with Drone Mobile auto-start, and a newly installed capacitor.

I have recently replaced the battery, since the old one froze and wouldn't hold a charge and I believe it froze because the alternator was not charging properly due to worn bearings. I also upgraded the charging system wiring to 2 gauge to accommodate my sound system.

Last night I got it started and began tugging on every bundle of wires I could possibly find (took 45 mins), paying special attention to wires leading to the PCM and the wires under the steering column and it made no difference in how the engine was running. The only thing that changed anything is that unplugging the MAF sensor increased the idle to a normal rate. Plugging it back in made no difference until the next time I started the vehicle...then it went back to 400 rpm.

Finally, this morning it wouldn't start on the first attempt so I turned the key to "off" and waited 10 seconds. I turned the key to "on" and the fuel pump primed and I was able to start the car 10 times in a row and drive to the store without it stalling. Since I didn't move anything between these attempts, I don't think it could be loose or broke wiring. Could it be the fuel pump control transistor on the PCM? Transistors are not mechanical, so it would make sense as far as how random this problem actually is. Should it attempt a solder re-flow on the PCM as a last resort? It seems like the warmer weather is helping the no-start to a certain degree, but a warm engine still seems to make it worse. Thanks in advance for the answers!

Doug_Scott · March 3, 2018

When it cranks but not start, have you tried pressing the gas pedal a bit?

TRUCK is about 14 years old now, unless someone at some point has cleaned the throttle body already it likely is loaded up pretty good, along with IAC assembly(assuming they still use that with drive by wire. No codes being set? I always thought idle speed was controlled by ECM based upon rpm signal. Idle speed being 400 and not being pulled up by ECM seems odd to me.

If the air inlet temp sensor is bad it may not be telling the ECM that it needs to inject more fuel to start the cold engine(computer version of choke).

cashflow50 · March 3, 2018

Clean the MAF. My 03 was doing the same. When slowing to a stop rpm dips below 400 then slowly goes up to 700.

Justin101714 · March 3, 2018

You could put the pcm in the freezer for about 15 mins and reinstall, see if you can duplicate the concern consistently.

You idle condition, make sure your throttle plate isnt full of carbon around the blade. Is this a drive by wire or does the throttle actually have a cable going to it?

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Nicholas McKay · March 3, 2018

I'm almost 100% positive has the electronic throttle. It's an 04 Z71... I think most V8's after 2000 or so has the electronic throttle. I'm pretty sure there is just an electrical connector going to the throttle body and no cable. Also, I basically live in a freezer at the moment. It has been slightly warmer for the last couple days (30 degrees) and if I were to put it in the freezer, I'm not so sure I could get it installed quick enough for it to maintain that temp. I haven't even attempted to remove it yet, since I couldn't get the cover open the day I tried...it was so cold that it hurt my hands then I slipped and hit the engine and gave up because that hurt like a SOB. I will give it a shot if the problem shows up again. I'll post later after cleaning the MAF sensor and throttle body and after flushing the coolant system (flushing the system since I have to partially drain the coolant anyway). I'll also check the thermostat while I'm in there so see if it's stuck open. Not so sure a bad thermostat would cause a low idle, but who knows...