I just bought a 2005 gmc jimmy 4.3L with manual transmission and 4x4. When floating gears like one does the rpm’s do not drop for 5-10 seconds. Is this normal? Any ideas what might be causing this?

 

No it is not normal. Sounds like it could be an issue with your Idle Air Control Valve. Is your Check Engine light on? Any codes? If not, before getting into diagnosis of the IAC, pop your air intake off the throttle body and clean it real well. Reset your IAC by turning the key to run for 10 seconds, then off for 5 seconds then immediately start the truck. But make sure the throttle body is clean first. I recently sent someone pictures on another thread, I'll find the thread and link it to you.

EDIT: Thread referenced is too long and doesn't relate to your issues. So I'm just adding pics to this thread. Purpose of paint brush is to keep throttle body open while you clean throat and plate as well as you can without getting your fingers caught. Clean it real well

 

I do have a check engine light on, came on after I posted this morning. I haven’t scanned it yet. Was going to do that later tonight. Thanks a whole bunch for the response!! I will be removing it and cleaning properly shortly. Is there a possibility that the cable might be sticking somehow aswell? I likely with clean both really good when I install a throttle body spacer in the next couple weeks here. I will post after I complete to confirm or deny if this was a good fix.

 

Sure it's possible, but I doubt it. I would think if it was sticking it would stick as well when you are in gear.

Let us know what code you turn up with. The freeze frame data would be good also if you can get it.

What are you putting a spacer on for?

 

****s and giggles mostly lol. I’m planning on throwing a cam and some other performance stuff on it too

 

Having been born and raised in SoCal in the 70's & 80's where we all hot rodded up our cars, I can fully appreciate that! Just be aware depending what you do you might have have a tune reprogrammed into your VCM. More power to you though!

P.S. Thank God I'm no longer in Cali! They ruined what was once an awesome state for car enthusiasts.

 

Went out and checked codes this morning, I have a p0420 code for catalytic converter efficiency less than 13%. Greattt

Nothing for Idle Air Control though. Could the cats be that restrictive that it won’t rev down? I haven’t gotten around to cleaning out the IAC, but will leave another update when I do.

EDIT: cats were going to come out anyway lol

 

There are two main problems that can occur with a cat: Its catalyst is exhausted or fouled causing it to lose its emission reducing function or it can begin to disinegrate and start causing excessive back pressure. Those two things can be separate or can coexist. All of this is usually caused by fouling from rich fuel, coolant leaks, etc. In an engine that's tip top, the cat usually lasts until the bone yard. This is why its essential to fix engine problems before replacing a failed cat because you don't want to kill the new, lower quality replacement.

That code tells you that the computer does not see the relatively flat line voltage output of the rear O2 sensor so the assumption is that the emissions function has failed. That may or may not be true. This does not tell you if the cat is plugged, the best test for that is a pressure gauge in the pre cat O2 port. A poor mans hack to check that is remove a precat sensor and see if it runs better. I don't think that this issue is a prime suspect for a stubbornly high rpm. While your down there you might want to check exhaust leaks which can freak out the O2 sensors. Others are helping you with you rpm problem so I'll stop short of directing traffic here but classic areas to investigate:

TPS live data readings at idle
fuel trims/unmetered air
O2 sensor graphs
Fuel pressure/leak down
IAC
Injector function
Engine mechanical problems

George

 

Ok. I looked into all of the above, I do have an exhaust leak, but it’s the flange after the downstream O2 sensor.

TPS- throttle position sensor(for those that don’t know) The scanner I have cannot rear throttle position at any rpm. Any recommendations for one of those? I do not want to hash out snap on or Mac money for one. Cheaper the better. Currently I have access to a nicer autel scanner but doesn’t have that function.

fuel trims/unmetered air- fuel trims look great, went through the whole air induction system, and it looks great.

O2 sensor graphs look good to me, hovering right around .785-.791v small fluctuations with high rev(when motor is cold) and normal operating temp. No issues there

Fuel pressure/leak down test- fuel pressure is phenomenal. The whole system operates as it should. Fuel filter is a little bit restrictive, will be changing that out soon. No concerns for me there

IAC- idle air control valve(for those that don’t know). Idle air control works good, idles right around 650, which is near perfect. When motor is cold idles up to 1100. I pulled the throttle body apart and cleaned then ran a bottle of ford motorcraft carb clean through it to make sure all the carbon and crap got blown out. Did not change my issue. But made me feel better.

Injectors- I pulled each one out and put onto a custom injector tester. Basically a hand pump with a t fitting. One side pressure gauge and other side injector. All were around the same, can’t remember what the number was.

Mechanical problems- had a misfire when I bought it, I redid the ignition system with AC Delco gold and/or gm genuine parts. Plugs, wires, distributor cap, and coil. Made a huge difference to fuel economy. It burns a touch of oil, but needs a oil change soon. Haven’t lost but 300ml yet. Coolant level hasn’t moved since I bought it.

Im starting to think it’s my TPS, if you could confirm my suspicion George that would be wonderful. Or any further things to look into?

Also, looking for a clutch, pressure plate, and slave cylinder. Clutch was bad when I bought it, and I’ve been limping it around. Recommendations on brand? I’m from Canada. Prefer not rockauto if possible

 

The way to test your TPS is to watch its value in % at idle running and then key on engine off as you move the accelerator pedal slowly back and forth through the full travel and watch for erratic or improper readings. This would be especially good during a high rpm event. There are various scanners that can look at TPS, the most cost effective (around $40) being the BT dongle/Android apps. Les can tell you which ones report this parameter. The usual suspects:

Car Gauge Pro
Car Diagnostic Pro
Torque Pro
Dash Command

There is also a comprehensive hardware solution if you are willing to cough up $150: Vident ilink 400. Above that are the stupid expensive scanners (ask me how I know) like Tech 2, Snap On, etc.


George