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Hard Starting and Rough Idle 1987 Celebrity

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    Hard Starting and Rough Idle 1987 Celebrity

    I have an 87 Celebrity with 2.8 V6. Saturday a week ago it would not start unless I pressed the gas pedal and than I had to stay on the gas pedal to keep it from stalling. After a minute or so it idles but somewhat rough. This occurs at start up every time now cold or hot. The car runs fine when out of the idle circuit. After checking everything according to the service manual I found a leaking fuel pressure regulator. Replaced it and I still have the problem.

    After running diagnostics on the computer the car's ECM thinks it is getting too much air at idle and is adding fuel when this is not the case. This appears to be creating a flooding situation and thus the engine will not idle until I give it air. I created an air bypass setup where I can add air without pressing the gas petal and this helps maintain idle until the engine warms up. After that the engine does idle at around 750 in Drive, but not smoothly. When de-accelerating the engine's RPM's vary as if the ECM is try to compensate for too much air, but does not stall most of the time.

    All sensors have been checked and are working within normal parameters. The Block Learn Values at idle with engine warm were running around 138 before I fixed the fuel pressure regulator. Now the are at 150 to 155. There is a code present, but it does not make sense. A code 33 is present (MAF sensor reading high GM/SEC). The Mass Air Flow sensor is seeing more air than is actually being applied, but according to the GM mechanic I saw seven years ago during a tune up this car's ECM has been updated and the MAF is no longer in use. If I was not told that this sensor is no longer used this could be the problem I think. But when I disconnect this sensor nothing changes, so this sensor should not be the problem or the idle would clear up when disconnected would it not? Any suggestions?

    Could the mechanic have been wrong or could there be something else causing this problem?

    IAC valve - Clean it up, and test it

    87 Celebrity with 2.8 V6 - Many of these were converted to a Speed Density setup, and a sticker would have been placed on driver's McPherson strut tower. The MAF sensor would not be used, just MAP and temp-sensor (engine temp, and maybe air-flow temperature via MAF). If engine temp sensor has not been replaced, it most likely is skewed downward in temp scale, which means more fuel would be added at startup/etc. Also, I can read my MAF's temp on a digital cluster...but, I don't know if ECU sees this or engine block temp sensor. Hence, if MAP or temp sensor(s) are screwing the pooch, ECU sees Garbage In, and puts the Garbage Out

    flooding situation - When peddle is depressed to floor, ECU has a preset program which kicks in.


      My Celebrity does have the Speed Density update sticker. It is placed on the passenger's McPherson strut tower. Engine temp sensor has been replaced about a year ago, but the scan tool has read it fluctuating in values. This explains why the transmission lockup goes on and off several times when around 160 until full operating temp is reached, since the lockup is temperature dependent. I may change back to the old one to see if the engine operates differently. For the last day and a half the engine has started up fine, and the scan tool reads normal IAC and BLM values until the engine is warmed up. Than the BLM values are up to 156 at idle and the IAC valve is back down to zero indicating the ECM is seeing too much air. Puzzling problem. Thanks for the tips.


        Engine temp sensor - There are two of for the idiot, and the other for the ECU; which one did you replace?


          I replaced the one for the ECM.


            scan tool has read it fluctuating in values - Question is, what?

            Chevrolet Engine Coolant Temperature Sensor Connector

            Grounding - I'd clean up ECU's grounds, and engine grounds.

            Many temp sensors are fed a fixed voltage, so, if one has a high impedance voltmeter, this voltage could be read on feed side, and return side to ECU.

            I've got a 1987 shop manual, but for Delta 88s, etc


              ECM is seeing too much air - Got a vacuum gauge?

              What is MAP indicating?


                BLM values are up to 156 at idle and the IAC valve is back down to zero indicating the ECM is seeing too much air - Is it smokin out the exhaust pipe? O2 sensor says?


                  Any suggestions?

                  Disconnect battery, for a hour:-) Reset Time


                    You have checked your EGR valve? And of course, you have a newer O2 sensor?


                      I do have the 1987 shop manual for this Celebrity. I will double check the connectors for proper ground and voltage.

                      MAP sensor is reading correctly according to the Scan tool.

                      Disconnecting the battery on this car will require a reset of the ECM for 15 minute upon start up to re-learn how to operate. A pain in my you know what.

                      The engine has been blowing blue smoke for a year an a half now. Spark plug check indicate cylinder No. 1 is consuming oil. O2 sensor is 4 years old and is fluctuating within normal parameters enough not to throw any codes. EGR valve has been disconnected for 7 years.

                      New theory: At idle do these engine have the throttle body opened at all, or are the throttle plates closed completely and idle is being maintained only by getting air via the
                      IAC valve?

                      I have the throttle plates opened slightly to maintain idle at about 750 even though the manual says it should be 550 RPM. It has been this way for years, but I think this problem has been around for years now that I think about it. It just has not been this bad that I must press on the gas to get it started, which by the way is getting better, but I do not know why. Over the years if I cut short the starting time (released the key from the start position too soon) that the engine did not start it would at up that I would have to add gas to keep it idling for a couple minute until it would idle on it's own. There are other weird idle things, but this explanation is getting long enough.

                      So if I set the throttle plates to be closed and readjust the TPS so that the voltage stays at .55v to .57v would this be the problem? Just an idea.


                        blowing blue smoke...O2 sensor is 4 years old - It's dead...$20.00...get it. Your Block Learn Values are based in closed condition, and used in Open Loop.

                        EGR valve - No vacuum lines attached? Spray some ether around it

                        reset of the ECM - I only do it when O2 sensor is fit for duty...1987-1988 Speed Density Software sucked here...1989 version was much better, but in Beretta, Speed Density was designed into MAF period.

                        throttle body - Use OEM specs....I cleaned one of mine several years ago, and I don't recall if closed or not at idle. On another engine I'm working with, which has idle air-bypass circuit, it is either barely closed, or is closed; I'd have to look to be sure.

                        OEM Specs - I'd use them...people who dink around with home-made "solutions" are fooling themselves...ECU expects a set of parameters to be fit for duty...there's the rub.

                        CTS Voltage - This needs to be nailed down...if supply voltage is changing, something is amiss in ECU...hence, a faulty ECU might be the real issue.


                          Is there a way of checking circuit voltage without disconnecting a connector to a sensor or switch? The Coolant Temperature Sensor (CTS) is in a hot place and difficult to disconnect and check while the engine is running.


                            Can you explain your statement "Your Block Learn Values are based in closed conditions, and used in Open Loop" in more detail. I know what closed loop and open loop mean. I know Block Learn Values refer to the amount of fuel the ECM is adding to the engine with 128 being normal. The higher the number the more fuel being added.

                            Thanks for your help.


                              checking circuit voltage - Maybe at ECU harness?

                              explain your statement - How GM Electronic Fuel Injection Works - Block Learn is a term that is related to closed loop mode, but continues its influence during all modes.

                              EGR - Get it fixed