This has a long sordid story. The problem started intermittently where the vehicle would barely start cold and I would have to keep coaxing the accelerator to keep it running for 5-6 minutes until it was warm enough to stay running on its own. I determined that it had the famous plenum vacuum leak and I could stuff a plastic bag down the throttle bores of the throttle body and it would still run. Installing a Hughes plenum kit fixed the mega vacuum leak. However... this made the problem worse. I find that if I pull the hose off the brake booster, the engine will start and idle fine. I have checked TPS, MAP and IAC motor. No codes are showing up on my scanner.


I replaced the TPS, MAP sensor and IAC motor for good measure as well as the coolant temp sensor.


Fuel pressure is 45 psi and is consistent.


Once the engine warms up and I plug the brake booster line back in, the vehicle idles and runs perfectly.


I am at my wits end. Any suggestions?

 

So is it just hard to start? Long crank time? Slow crank? Will it start if you give it just a bit of gas pedal?

 

Long crank and struggles to start. Wont stay running unless i get the engine to at leat 1500 RPM. Foot off gas and it dies. Pull the hose to the brake booster and it fires right up and idles fine. Acts like the open loop presets in the ECM/PCM are corrupt. IAC is new. Throttle body recently cleaned.

 

What brand IAC? Did you reset the PCM? Yanking the booster hose just opens up a big vacuum leak, the engine can draw more air (bypassing IAC), so it will run. Since it runs fine once warm, that leads me to think that either the IAC isn't working properly, or, the control circuits have some issues. (which would usually set a code.... but, not always....) I had an 'off brand' IAC that gave me the same issues. Installed a Mopar part, and problems went away...

 

Bosch IAC. Yes... I reset the PCM with my Snap On Scan tool. I think I will warranty return the IAC for starters.

 

Does that completely clear the PCM 'learning' process?

What you want to happen is, for the PCM to forget everything it learned previously about running the engine. That forces it to 're-learn' the zero positions of various sensors. (chiefly, the IAC, and TPS.) The first time you start it, the truck closes the IAC completely, to find where that point is, and checks TPS to see what voltage is at what it assumes is closed throttle.

I like to disconnect the battery for a few minutes, (turn on the headlights for a short time, to completely drain all capacitors in the PCM) then, turn the key to "on", (not start) count to ten, so the PCM has PLENTY of time to figure things out, THEN start the truck. usually, it fires right up, and idles on its own first try.

Not sure how I feel about bosch, but, I know my truck really disliked the "duracrap" IAC I put in there, and ate it in about seven months......

 

I cleared the memory with my scan tool. My Snap-On Solus has a battery disconnect clearing process.

 

Ah, the expensive toy. I like those. Very handy.

Getting 12 volts on two of the wires to the IAC? If ya are, I think swapping it out will be the hot ticket.

Won't the Solus manually move IAC? Tried that yet??

 

it wouldnt happen to act like its flooding would it? my 98 dakota would do the same thing where i had to hold the gas until it was warmed up or else it would die. when it would die i got a strong smell of gas and had to hold the pedal to the floor to get it started and sometimes i had to let it sit for a few minutes. turned out to be my intake air temp. sensor, when it was cold it would read crazy numbers like -50 degrees, so i think thats why it would "flood" (funny thing to say about a fuel injected engine). Long story short i changed it on a 0 degree day and the truck fired right up and didnt die. maybe take your scan tool and see what that sensor is giving for a reading when the truck is cold? not sure if this will help, just kind of sounded like my dakota when it was cold.

 

Victory! It turned out to be a bad(new) IAC motor. It was a Borg-Warner (not a Bosch) that I had replaced only about 2-3 weeks ago. When I went to remove it the pintle was all the way out and looked like it was jammed into the hole in the throttle body. The identical BW replacement works like a charm. Idles perfectly.