difference between open loop and limp mode
Thread Starter
Instructor
Joined: Dec 2004
Posts: 105
Likes: 0
I'm trying to isolate a problem with my 85. MAf unplugged the car runs o.k. (i assume in limp mode). When i plug MAF in the car runs really rich and the car runs terrible (barely starts). This happened all of a sudden. I thought it was the MAF and i replaced it (even took it back and got another one). After doing searching i believe a sensor is not working. I dont have any meters or diagnostic tools so i'm trying to minimize the testing. no codes are showing so I'm trying to figure which sensors arent used in limp mode to narrow my search.
I was thinking it was the tps but since you say its used in limp mode maybe its not that.
Does anyone know if the cold start injector is used in limp mode? Or any other input on what is used on limp mode.
I was thinking it was the tps but since you say its used in limp mode maybe its not that.
Does anyone know if the cold start injector is used in limp mode? Or any other input on what is used on limp mode.
Melting Slicks
Joined: Apr 2003
Posts: 2,150
Likes: 6
From: NS
Originally Posted by batoutofhell
What sensors are used in open loop that are not used in limp mode. thanks
EFI 101...
When you unlpug the MAF the PCM/ECM will control the engine based on the fuel tables. It mainly uses the MAP sensor and RPM with the TPS (for transitional fuel i.e. pump-shot) and IAT/CTS/BARO for density calculations (fine tuning the fuel tables).
For your problem...the MAP/RPM combination allows the PCM to look up the engine's efficiency from a table it has in memory (calibration). Knowing the efficiency, and air density, it calculates the volume of air entering the engine. This is known as a speed density system. With a MAF sensor, the MAF simply measures the volume of air entering the engine and reports it to the PCM.
My guess if you have a bad MAF. It's telling the PCM more air is entering the engine than really is, so the PCM injects gobs of fuel as it tries to hit it's A/F ratio target.
Le Mans Master
Joined: Apr 2002
Posts: 5,726
Likes: 46
From: Waukesha WI
St. Jude Donor '15
Might also want to check the MAF relays - they can emulate the symptoms of a bad MAF. You can check these with a test light and the wiring diagram (I don't have on for 85).
could be the two relays (at least an 87 has two) there cheap,
