03 Ram cold start problems.

03 Ram 1500 4×4, 4.7 engine, automatic transmission.

It can start hard and has erratic idle, stalling, and stumbling troubles on cold starts and I generally get no more than two minutes to try and diagnose it from the moment I crank it on cold starts only. If I try to drive it right away, it can hesitate as if it’s choking out like an old quadrajet carb on take off. In tight parking lot maneuvers, I can usually stall the engine out with little throttle bumps trying to get the truck rolling.

No codes stored so I went to live data and tried to look at IAC data. I didn’t get too good a look at data, but I did notice a few moments that it looked like the IAC percentage wasn’t really moving on throttle and it didn’t really look like it was compensating when letting off the gas pedal until it warmed up. I was really fumbling with the scan tool so I lost a lot of data I’d have loved to just show you.

I found an actuator test for the IAC so I moved forward on the diag anyway. The scan tool gives me a range to move idle from 800 up to 1500 rpm in 100 rpm increments. The first time I moved through that rpm band, actual idle speeds would fall short of commanded rpms. At 1100, I got 1060, at 1200, got 1150, and ultimately, 1500 commanded never got past 1250ish. I ran through the commanded speeds a couple times and got some repeatable results with some deviations with higher actual rpms. In one particular instance, as I was commanding idle speeds back down toward 800, the actual rpms were 40-50 rpm over commanded instead of being short on rpm. I went through the command speeds again; this time knocking on the throttle body/IAC housing area with an extension. A few times I got high idle spikes toward 1500 rpm at the 1100 and 1200 commanded speeds. Often enough, rapping on or by the IAC would let me match actual idle speeds within 15 rpm of commanded rpm. This happened between the 1100 through 1400 command speeds, but the truck still wouldn’t rev up to 1500 when commanded. The best I could do was low 1400s. I had a scope on the IAC to monitor pulse width and it would change as idle speeds were increased or decreased. The wave form was missing a symmetry I was hoping to see, but the wave form was consistent. I’ll try to get a snap shot of the wave form tomorrow. It’s something like one fat pulse followed by two pulses at half the width of the first.

At this point, I felt good about replacing the IAC after cleaning it didn’t really change anything. I went through the rpm band again and I still couldn’t get actual rpms to match commanded rpm. Right now, I’m stuck until morning to see if my diagnosis was right, but I’m skeptical given the last actuator test.

Other things I don’t know are important, but I’ll tell you about them anyway. My Modis did the scope work at the IAC, the shop’s Solus was connected to the DLC for scan tool work. The shop uses Identifix and it gave me factory wiring diagrams that suggested my IAC was a stepper motor design. It is, in fact, a PWM solenoid design. MAP sensor readings appeared normal during testing. I didn’t get a look at any O2 sensor pids and I haven’t checked for any fuel problems yet. I did a tune up on this truck about a month ago and I thoroughly cleaned out the throttle body and IAC valve at the same time. The truck had slight stumble/near stall on those throttle bumps once in a while back then. I didn’t chase that problem then because the owner just wanted a power loss problem solved. Hence, the tune up. I was told to smoke test for an intake leak and I did that with no intake leak to show for.

What did I maybe miss? What would you do different?

Thanks