Mitchaul

I have been driving my C4 with the Ses light on for a couple of months and the car was just fine, idle was good and great accelerations.

But one month ago it was wery hot outside and the engine was over 110° Celcius. When I pressed the gas padel, the engine did not react right away, it seemed as if there was a lack of petrol and after 2-3 seconds it would run fine. Whenever the outside t° is high I have the same symptoms. Any suggestions ?

I v'e already unplugged my MAF, cleaned it together with the trottle body, changed both relays and unplugged the battery but I still have the SES light with code 34 (Maf sensor). What's next step, change MAF ?

Thanks for your advice !

MrWillys

http://www.chevythunder.com/maf_code_34.htm

Mitchaul

Thanks for the link Mr Willis ! I went to a meeting with the car today and when I drove back it was even worse. The car would start as usual but was shaking when I was accelerating and i had the feeling of power loss ! Is this related and getting worse or is it just someting else added ?

Thanks again in advance for your help !

Mitch'

Cliff Harris

Try unplugging your MAF and running the car. You should get an error code 33 (MAF voltage high, which is correct because the ECM pulls the MAF output to 5 volts internally). If you still get a code 34 then the wiring is suspect (probably a short to ground on the MAF signal wire).

Mitchaul

Well I did unplugged the MAF and I do have code 33, good news I guess. Does this mean the MAF is not faulty ? What is the next step now ?

Thanks again for your concern

leesvet

The ECM has 2 'modes' it will operate in....

when normal its in whats called the CL mode after its warm and everything is ok. Closed Loop.

If there is a problem from a faulty sensor or some input that's out of the normal operating parameters it drops out of CL and back into OL...Open Loop mode or what some refer to as LHM...Limp Home Mode.

CL is the engine running of of steady input from the sensors...
OL is the engine running off of stored programs and some stored data in the ECM that's used for this reason, to be able to run the engine well enough to get you home. It may not pass smog but it will run. That's LHM as well...

If your MAF or some other sensor is bad and you keep that plugged in, the ECM will set a code to indicate that this or that sensor has issues. It DOES NOT mean that sensor is bad. It means that sensor circuit has issues of some kind or that its been effected by the fault.
It will continue to run crappy as well. For some reason, some cars will be reluctant to make the shift to LHM and they will run like crap..backfiring, will not take throttle etc...

You can force it into LHM by starting it with the MAF unplugged and keeping it running for about 10-30 seconds after its warm and the ECM will go to or stay in OL and run the engine off the Cal-Pak (calibration package)...the stored data or program so it knows what to do.

IF you already tried to run it with the MAF unplugged and it STILL ran like crap...
go directly to the FUEL system and change the filter and pump after getting an accurate fuel pressure test showing less than 40 psi when the key is turned on but not started...

The fuel system is the only critical system on this engine that can fail and NOT set a code...The codes you are seeing may be coincidence. If it runs fine with the MAF unplugged, its certainly electrical in nature. If that had no effect then its fuel system.

leesvet

'''

Mitchaul

Thanks for this complete information Leesvet ! Regarding my ses light with code 34, it is on since I changed my lambda probe. I had to get one that gives fake information in order to pass pollution test here in France. I don't think it is related to the problem described on my first post.

65Z01

What is a "lambda probe"???

Often when a problem exists following some work done, look in the area of that work for something that was disturbed.

Clear the codes, start the engine and see if code 34 is set within 1-2sec. If so do a voltage check on the TPS sensor; the sensor wire should read ~.6-.8Vdc at idle.

If the TPS is ok, with MAF disconnected and engine running, probe the Ppl wire between MAF Power relay pin E and MAF Sensor pin E for +12Vdc.

If +12 Vdc present replace the MAF Sensor.

BTW, if you know someone who has a 3rd gen F-body TPI car ('86-'89) with a good MAF Sensor you can swap that in to see if it fixes your problem.

Bonne chance.

Cliff Harris

Limp mode is completely different from Open Loop mode.

The ECM starts in OL mode and switches to CL mode when various conditions are met, mainly that the O2 sensor gets hot enough to put out a valid voltage. WOT will also put the ECM into OL mode (it can't calculate fast enough to maintain all the CL functions). The main difference between Closed Loop mode and Open Loop mode is the ECM does not update the BLM cells in OL.

In limp mode the microcontroller is turned off and the FMD (Fuel Modeling Device) chip takes over and runs the engine using the resistor networks in the MEMCAL. It only uses the CTS and TPS to guess how much fuel to put into the engine and the DRPs (Distributor Reference Pulses) to synchronize the injectors to the rotation of the engine.

There has been a lot of speculation (but no hard info -- like from GM) on what puts the ECM into limp mode. I recently saw a post by Robert Rauscher where he thought a battery voltage lower than 10 volts would cause it.

Cliff Harris

Another name for oxygen sensor.

It actually measures lambda, which is the ratio of the actual AFR to the ideal AFR.

An interesting factoid: it doesn't matter what kind of fuel you're using. The lambda sensor will always read lambda = 1 if the actual and ideal AFRs are equal. This becomes important if the gasoline has ethanol added because the ideal AFR for 10% ethanol is 14.5:1 vs. 14.7:1 for pure gasoline. This also works out well for us if you add fuel injector cleaner or other "mechanic in a bottle" chemicals to your gasoline, as they will affect the ideal AFR.

leesvet

Actually its 9 volts or less @ the plug OR 9V to the starter.

Yes Cliff, Thank you for pointing that out. I am grateful for your vigilance in pointing out the technical errors in my post....

However,

I am WELL aware that LHM is different than OL.
LHM is just info stored in memory. (Cal-Pac)Calibration package as a reference for run parameters.
Cal-Pac offers min info to replace sensor input when it is not available or certain other ECM interruptions take place.

OL is the normal mode that is in operation (run)with an incomplete sensor loop (all reporting to complete this loop, thus the term Closed loop). The o2 is the secret ingredient to CL. Fuel is metered via TPS instead of o2 during OL.

Thanks again, Cliff. If you keep in mind that we're (I was) attempting to help someone that is unfamiliar with all of this, I was TRYING to keep it simple.

You'd better take it from here.

Cliff Harris

Hi Lee. I didn't mean to hurt your feelings or anything. Based on the tone of your post I get the feeling you're a little upset. Would it help if I reminded you that I sent you an C68 ambient temperature sensor a few years ago (for free)?

I'm just trying to help, as you are (I see how many stars you have under your username ;-). I just want to keep the facts straight and avoid the spread of misinformation. We have WAY too much of that.

I saw a post recently that said disconnecting the MAF or the O2 sensor would put the car in limp mode -- it won't. That kind of info I'm trying to keep off the forum.

65Z01

Thanks for the tech def. and the info on its operation...

But I'm a bit confused as to how this relates to our TPI cars. My scanner shows "cross counts" in closed loop mode, i.e. O2 sensor warm and value varying around the 0.5 mark. When I go to WOT the O2 reading goes to a steady ~0.91-0.92 range???

Mitchaul

Well, thank you all for your help and your care about my C4 ! I will check what has to be checked and will come back to you hopefully with good news

Mitchaul

Today I've disconnected the MAF and the car would run much better. I think changing MAF would solve the problem. Which MAF would you recommend ?
Among parts provider here in France, one can get me a STANDART MAF.
Anyone has any feed back on this brand ?

Thanks again for your advice.

Mitchaul

Today I've received my new ACDelco MAF and.... no more code, no more troubles driving !