oldparts Posted February 16, 2017 Report Share Posted February 16, 2017 I just bought the cutlass without a smog, It failed nox at 1100 both at 15 and 25oo rpm. It ran fine, but under a load at 55mph it felt like a boat...surging and idle fluctuations...I figured it was the cat///replaced it /// So I replaced the MAP, O2 sensor, Coolant Temp. sensor, Evap canastier, power brake check valve vaccum line, fixed small vacuum leak...no avail When warmed up, idling, and just giving a little gas...... it stumbles and dies...sometime can rev up to 2000 rpm then dies Has anyone had this problem? ....it ran better with the old parts...never did this until I put new parts on.... Quote Link to comment Share on other sites More sharing options...
Nas Escobar Posted February 16, 2017 Report Share Posted February 16, 2017 I'd check out the fuel system to make sure you're getting adequate fuel to the rail, but from experience, I'd suspect the fuel injectors. The 2.8 and 3.1 MPFI were not known for good fuel injectors. It's actually one if the MPFI's folly. You can rent a fuel pressure tester tool from the local parts store to test how much pressure you're getting at the rail. The test would look something like this... That would determine that your fault is between the fuel pump and fuel pressure regulator. To test the injectors (assuming you have normal pressure), you're gonna have to take the intake plenum off. Your fuel injectors may be heat soaked and need to be replaced. You'll have to check all 6 injectors with an ohm meter. If you get a reading lower than 12.0 then that injector is bad. In my experience, I had an 89 Buick with the 2.8. 4 injectors were bad. One of the fuel injectors had an ohm reading of 3.8, and only 2 were 12.0. That car would cut off when at running temperature or when revved up under pressure (think uphill). Quote Link to comment Share on other sites More sharing options...
oldparts Posted February 18, 2017 Author Report Share Posted February 18, 2017 Thank you for your expertise...Would bad fuel injectors then be causing a lean condition and therefore causing a High Nox of 1100. both at 1500 and 2500 rpm....this is why I thought plugged cat, and replaced it, it wasn't plugged or muffler either... I am getting a mechanic with a GM Tech 1 scan tool for Idle Relearn, sunday....I did get a big fat service manual for 1992 cutlass supreme and it does state...GM TECH 1 Idle Relearn if battery has been interrupted, and I did get another battery... So the car runs ok starting and while cold, then when injectors get warm the injectors heat heat soaked and loss resistance...is that right? Quote Link to comment Share on other sites More sharing options...
Nas Escobar Posted February 18, 2017 Report Share Posted February 18, 2017 Thank you for your expertise...Would bad fuel injectors then be causing a lean condition and therefore causing a High Nox of 1100. both at 1500 and 2500 rpm....this is why I thought plugged cat, and replaced it, it wasn't plugged or muffler either... I am getting a mechanic with a GM Tech 1 scan tool for Idle Relearn, sunday....I did get a big fat service manual for 1992 cutlass supreme and it does state...GM TECH 1 Idle Relearn if battery has been interrupted, and I did get another battery... So the car runs ok starting and while cold, then when injectors get warm the injectors heat heat soaked and loss resistance...is that right? Can bad fuel injectors cause a lean condition? Yes. When an engine runs lean, it means that the air to fuel ratio is biased towards air. Not a lot of fuel getting to the cylinders. Can it cause high NOX? I'm not sure. I do know that your readout is supposed to be indicative of what the issue is. It may be of no use to do idle relearn if you have bad injectors. You'll have to do the idle relearn again once everything is in good condition. If your engine runs fine when cold, and you can even run it under high load when it's cold, but it stumbles and won't run when it's hot, the fuel injectors are heat soaked. Yes they lose resistance but the fuel injectors may be bad to begin with. That was the case with my 89 Buick. Only 2 injectors were good and it would run nice and smooth despite this but it had no power whatsoever going uphills and it would cut off 15 minutes after running. Less if I cold started it and drove it. If your issue is the same as what I described in my 89 Buick, it's fuel injectors. It wouldn't hurt to double check them with an ohm meter, but I'm 90% sure you have bad fuel injectors if it cuts off around 2500 - 3500 when the engine is under load. Quote Link to comment Share on other sites More sharing options...
Schurkey Posted February 21, 2017 Report Share Posted February 21, 2017 Schurkey's Perpetual Advice: You need a scan tool so you can view live data. Nothing else will be as effective or efficient at diagnosing the problem. You also need to verify fuel pressure at the fuel rail, while driving (under load.) High NOx? If I were lacking scan-tool data, the first thing I'd look into is the EGR system, including the wiring and the EGR passages in the intake and exhaust manifold. NOx does not form below a certain temperature. Anything that causes high combustion temperature causes NOx. This could include faulty cooling fans, faulty water pump, lean fuel mix, etc. Quote Link to comment Share on other sites More sharing options...
oldparts Posted February 23, 2017 Author Report Share Posted February 23, 2017 Got a mechanic with a old obd 1 scan tool.....not to many of them around. Found Mapp sensor defective, the one I just replaced due to overhauling emission system... Did not use GM Tech 1 tool after reconnecting battery, but did idle relearn as idle, till 160 degrees, turned on AC, no difference in load...turned off....then put parking brake on and shifted to drive...back disk brakes held and idled at 500 rpm...shifted to Park, idled to 800, then revved to 2,500 slowly and it hesitated slightly, then did this 3 times slowly, and after that it was working fine. I haven't takin it out on a drive, tomorrow I will before taken it back for retest on smog. Quote Link to comment Share on other sites More sharing options...
oldparts Posted February 27, 2017 Author Report Share Posted February 27, 2017 Passed smog NO2 1500= 400 2500rpm= 40...now I didnt get a certificate because I accidently put on a magnaflow P2 on a P1 car...but it passed and runs better, even the low rpm in gear and stalling is now happening.... Quote Link to comment Share on other sites More sharing options...
Nas Escobar Posted February 27, 2017 Report Share Posted February 27, 2017 So the car is still stalling? Does the certificate affect anything? Quote Link to comment Share on other sites More sharing options...
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