low cold start idle(IS BACK!!)

I am currently looking at a 1995 Toyota Tacoma 2.7L 4x4 that's in great condition for its age and the body has 137000 miles and around 80,000 miles on the swapped engine. It was swapped because the previous engine had a blow head gasket. Anyways it is running really well but the only issue I have is on cold starts the engine idles low around 500 rpms or less but if you let it warm up after a couple minutes it runs and idles fine but if you take off as soon as you start it cold theres some hesitation. Any insight on my situation?

Things the owner has changed/checked:
-egr valve
-egr solenoid
-egr temp sensor
-egr vacuum switch valve
-spark plugs and wires

P.S. all these things that were checked/changed was because it previously pulled a code P0401 Insufficient EGR Flow but the code is gone now but other than that the low cold starting idle doesn't bother him as much as it bothers me lol

 

It could be at IACV (Idle Air Control Valve) or the MAF needs to be cleaned. Check for cracked exhaust manifold, you will need to remove the heat shield, but once you see it you will know. A very small hole in a vacuum line can be tough to diagnose, but can cause idle issues also (among a host of other things).

Edit: the IACV is very expensive (+$200), so I would clean the MAF and check for vacuum issues before going that route. When I rebuilt my 2.7 I went ahead and replaced it, but I would only do it if I had checked everything else. My guess would be the MAF is dirty. Once the engine warms up the Intake Air temp will climb up a bit and the MAF will send the data to the ECU, which can increase idle, etc.

 

That's what I was thinking too but I wanted to make sure I ruled out all other culprits before I jump to the IAC Valve. A good cleanining could possibly be all it needs. I'll look into the exhaust manifold. I was also planning to just replace all vacuum lines just to be safe anyways. What would be a easy or good way to detect a vacuum leak though? And how likely would the problem come from a faulty Intake Air Temp Sensor?

 

A dirty MAF sensor or vacuum leak usually throws a P0171 code but it can't hurt to rule those out, they're very easy to do yourself. Get some MAF cleaner (not carb or electrical cleaner, MAF specifically) and give it a cleaning, see if that helps your idling issue.

When I cleaned mine it idled funny for a few minutes after starting up but returned to normal and was fine from then on. Just a heads up

 

Eon is dead on. Normally P0171 will appear when the MAF is dirty or a vacuum line has an issue. I would still start there and proceed before throwing parts at it.

 

Cleaning the MAF Sensor is definitely on the top of the check list. Thanks for the insight guys. Will be giving updates on the way.

 

That symptom is very likely to be the IAC being sludgy. Keeping the maf ckean is always a good thing to do but my monies is on the iac... if its never been cleaned it will certainly be a lil gunky inside there. Clean your throttle body while youre at it, it'll be dirty too!

 

Alright guys so I cleaned the maf sensor,throttle body, and iac valve. Replaced the air filter too since it was really dirty and all the gaskets. It idled better on the cold start but we'll really have to determine if that did the trick tomorrow morning. But the weird part is after all that cleaning the idle was high. Before it would idle between 650-750rpms but now its just a little higher than a 1000rpms, any insight on that?

 

Totally common for a high idle after a good cleaning. After ya do a couple few drive cycles it should settle right back down to a normal rpm.

 

I've read that in a bunch and forums and posts. Hopefully that's it, and by a couple drive cycles do you mean a couple of days? After a good cleaning today I took it around the block a little and took it about 17 miles to the other side of the island and back later on that night. I'm a little impatient sorry lol

 

It will come back down to normal. It will vary on time, but the ECM/ECU is getting updated info from the sensors. I would drive it for a while (few hundred miles) before I started worrying about it idling high. As long is it isn't throwing any codes or idling rough again.

 

My idle was high after cleaning my MAF but it returned to normal after driving around the block for a bit. If you cleaned your throttle body as well it might cause the high idle to hang around for a bit longer but it should return to normal in time.

 

Ok, hopefully after a couple runs it returns to normal. I started her up this morning and the idle dropped to 500rpms right after starting but quickly went up to 750 after a couple seconds but it slowly rose to a 1000rpms. Before I did a good cleaning during cold starts it would linger around 500rpms sounding like it's gonna stall for about less than 5 minutes then it would climb back up to 650-750rpms. Don't wanna jump to conclusions but how likely would it be that I damaged the iac valve during this cleaning process? Could it be stuck open causing the idle to stay high? Just a food for though.

 

Ok guys, i'm convinced that this idle isn't gonna get better until I fix or replace something. At cold starts(in the morning) I still get a quick low 500rpm idle for a couple seconds then it slowly rises 1000rpms. After running it for a bit or even just revving the engine a little, the idle rises to no more than 1250rpms and doesn't come back down at all unless you shut the engine off and crank it up again. I'm gonna remove the throttle body and iac again and give it a deeper clean and try to check resistance like a should've the first time, my mistake lo. Quick question, so the part that opens and closes the throttle body butterfly and also holds the accelerator and throttle cable, is the throttle stop screw and lever supposed to be touching eachother when the truck is running? I noticed it touching when the truck is off but theres a gap when the truck is on. I don't know if this is normal or not and if someone can assure me. Thanks guys for the help!

 

So a month or 2 passed and the rough/low cold start idling issue resolved but I'm still left with a noticeable high idle at 1000-1250 when warm. Help anybody? Looked for any vacuum leaks and I'm yet to find one yet. The weird thing is that it runs totally fine besides having bad gas mileage due to the high idle.

 

Fixed the high idle after speaking to a member of LCE Engineering. Happened to be the Engine Coolant Temperature Sensor. Pulled that PITA right below the valve cover right underneath the wiring harness on my 2.7l engine. Had to remove the whole air intake tube so I could move the wiring harness to the side and reach all the way in the back. But anyways, it kinda takes a while for the idle to settle to 700-750 from a cold start but most likely because the thermostat is sticking a little. Was supposed to change it with the coolant temp sensor since I had to drain the coolant but the local parts store didn't have the gasket so decided to replace it another time.