94 Q45t cold start idle problem

[Razi]

Our 94 Q45t had idling problems and it's rpm would go from around 750 down the 250 and back up again. And the CEL came on recently and we got the codes pulled, and it had problems with the MAF, Knock sensor, and EGR.

We cleaned the EGR valve, and replaced the MAF sensor, now once it's warm it does not have bad idling problems. Also, probably unrelated, our old fuel filter got replaced also.

Yet when it is cold it's idling problems stay around for a few minutes until it's warm, and after 20 minutes of driving the CEL light comes on.

If you guys need to know anything else, ask me.

[maxnix]

No mention of IAC valve or TB condition?

Any other fuel system or intake path maintenance to recount for us?

[Razi]

TB has been cleaned out, same with PCV valve.And the IAC valve has been untouched.I ohm'd all 8 of the injectors, and nothing seemed wrong.

Edit: Perhaps me and my father can clean the IAC valve out, I'm not looking at it right now but from what I remember, it seemed tricky to remove. Is it just a straight forward turning of my wrench and pulling out the IAC valve?

Also, I know some Q's this age had bad fuel pumps, but that was replaced a long time ago, and no more buzzing in the trunk.

When we shut down the engine, and start it up again, the CEL does not appear again for 20 minutes. And,randomly the CEL does not come on during the whole commute.
Modified by Razi at 6:56 PM 8/31/2008

[goody90q45]

.......Perhaps me and my father can clean the IAC valve out, I'm not looking at it right now but from what I remember, it seemed tricky to remove. Is it just a straight forward turning of my wrench and pulling out the IAC valve?

Before you start have a new IAC gasket on hand. Loosen and remove the four 10mm bolts that hold the IAC to the plenum. I use a 1/4" drive ratchet wrench with short extension and a 10mm socket. Removing the 4 bolts first will allow you to pull it back a couple of inches to unplug the connector and easily get to the hose clamp on the bottom of the IAC.

When we shut down the engine, and start it up again, the CEL does not appear again for 20 minutes. And,randomly the CEL does not come on during the whole commute.

You need to read the codes again and see what's causing the CEL to come on. Probably best to leave the ECU laying on the floorboard for a few days so you can easily check for codes and reset it a few times. Many times the 20 minutes after cold startup CEL will be EGR related.

[Razi]

Thanks Maxnix and Goody! I love the Q45 forum people

[Razi]

Guys, I have cleaned the IACV but the problem is still occuring.I'm out of ideas as to why this is happening, the car has also stalled this morning at it's 1st stop sign.

[3Q Jay]

i missed if you have done the full plenum cleaning or not. assuming you have no intake leaks, i'd first check the aac setpoint. consult would be best, but oif you don't have it, then start the car, get it warm, disconnect the aac electrical plug, and use the air bypass screw to set base idle to 600 in park, no accesories. then when you plugit back in, you should get a quick rpm surge and hav eit settle back out in ecu control to 650.

[Q45tech]

As engine ages, wears, loses as new peak compression, it may be necessary to raise the warm idle rpm a little to maintain smoothness.

Remember a steady rpm idle is the point where there is ZERO extra HP/lb-ft otherwise the rpm would increase.

Data on individual cylinder compression test is useful to calculate new used rpm or just try 25 rpm increments until it is smooth after 15-20 minutes of running.

[Razi]

No I have not done the full plenum cleaning, and Q45tech, I understand what you are saying and thanks for the input.

But one thing I forgot to mention, I don't know how I left this out.

This problem started occuring a little bit after we cleaned out the EGR valve, but as I understand it, the EGR valve does not engage during idle, and it only engages when the car is warmed up and driving around. Yet the car runs fine, only the idle gives us problems.

[maxnix]

Did you check or clean the EGR down tube?

The EGR inlet on the plenum is basically impossible to clean with the plenum installed.

[Q45tech]

Mini Rotor Rooter [made from old throttle cable] and drill.........remove EGR valve assembly first.

[Razi]

Thanks, I will try to clean the EGR downtube tonight.Will report back tomorrow morning.

[Razi]

Well, we just found out the problem, it is a cracked vacuum hose under the plenum.

My guess is, when it is warmed up the rubber hoses expand and shrink the crack and the idle evens out, but the opposite happens when it is cold.Since it is under the plenum, I see a big job in the future.