1999 Chevrolet Monte Carlo

There it is the 1999 Chevrolet Monte Carlo with 128,191 miles with a 3100 motor that seemed posessed. The car was towed in for a no start. The shop diagnosed it rightfully so as a bad fuel pump. Customer decided to supply their own parts. Shop installed customer supplied pump and car leaves. Car comes back a couple of days later with fuel pump belly up. Shop owner convinces customer to install a quality fuel pump assembly instead of best price unit. Customer agrees, pump is installed and car leaves. Car comes back with complaint of lack of power. Fuel pressure is at spec. Shop takes a look at load under WOT (Wide Open Throttle) and it is barely 60%. We like to see 80% or better on a naturally aspirated vehicle. In went a remanufactured Mass Air Flow sensor and now we have 82% under WOT conditions and the car is thoroughly road tested. Car leaves and customer returns a couple of days later thoroughly incensed. Now the complaint is the car is cutting out, runs bad, etc. This is where I come into the story. This shop owner is pulling his hair out. Do we have a defective remanufactured MAF sensor? Another bad fuel pump? The shop owner let's me road test the car. So I hook up my Tech2 scantool arm it to take a snapshot and head out. This car runs rather well. I sneak a peak at the load and MAF pids under various driving conditions and they seem right on. I road test the vehicle close to 12 miles under every concievable driving condition and it runs great. Back at the shop I pop the hood and the first thing I notice is it looks like it has the original ignition coilpacks and ignition wires. Hmmm. I grab my spray bottle of salt water and mist away. It wouldn't be the first time I have seen poor secondary components causing this issue. This car doesn't miss a beat. So I hot soak it for a half hour still nothing. I low amp probe the coilpacks and the injectors-nothing! The shop owner agrees to take it on an extended road test. The next morning he calls me and tells me he got it to act up after about a 30 mile road test. I tell him I will be by later for another road test. Again, I arm my Tech2 and go for an extended road test. After about 20 minutes of driving I feel a sputter. I immediately hit my record button. It sputters some more. This is a hard shudder. Like someone shutting the key on and off.

Back at the shop I download my snapshot. I choose to look at 5 pids closely. Engine Rpm, Cam Signal, Ignition 1, 24x Crank Sensor, and Spark Timing. This 3100 motor has a 7x Crank Sensor in the block that reads off notches on the crankshaft. This sensor is the engine speed input to the Ignition Control Module. From there the module then conditions this signal and produces what is called Ckt. 430. This is the main rpm input to the PCM for engine speed calculations, ignition timing, fuel injection control, etc. We also have a 24x crank sensor that reads off the front of the crankshaft via windows on crankshaft dampener. This signal is sent directly to the PCM. The PCM uses this signal for low speed timing events. Typically this signal stops being looked at above approximately 1600rpm. Then there is a Camshaft Sensor which is utilized for sequential fuel injection. Every year has different wrinkles for each of these sensors. What I layed out is a generalization. So, if we look at the snapshot above the first thing I see is that 24x is above the usual rpm limit of approximately 1600rpm. It is at 2184 rpm. The other pids look ok. Let's go backwards in the snapshot.

Here you see typically where I see 24x stop incrementing. Looks go back even further.

Here you see that Engine RPM and 24x are under 1600 rpm and are within 50 rpm of each other. This is when the car is running well. Lets look at when this car was really having issues.

Here we have a real issue. The Engine RPM is under 1600 rpm but 24x is above. What gives here. I have seen weird things happen with 24x sensors as well as Cam sensors. Thankfully, most of the time you can just unplug the offending sensor and road test to confirm if that is the problem. So, at this point I feel pretty confident that after I unplug 24x sensor this car is going to be alright. So, off I go figuring I am done here. Well the car still hiccups and runs poor. Feeling dejected I plug the 24x sensor back in and decide to look at the 24x rpm counter when it hiccups. Sure enough every time it hiccups it gets a stupid rpm increment. There are times that you fix a car more with your gut then a scantool or a fuel pressure gauge. With this in mind. I hook my scope to the 7x Crank Sensor and CKT. 430.

Here it is 7x on Channel 2 and CKT. 430 on Channel 1. I didn't even make it out of the parking lot when I got this. We have nice 7x but no 5 volt square wave on CKT. 430. Here is another one below out on the road.

There is the money shot! This is during a hiccup. We have proper signal into the Ignition Control Module but improper out. We are missing CKT. 430 pulses. I torture the modules power and grounds to make sure we do not have a power or ground issue. This car needs an Ignition Control Module. I advise the shop owner that the original secondary components should be replaced with the module as well. As far as the scantool data. I am thinking that when there is a CKT. 430 issue the normal rpm increment for 24x goes out the window. I know for next time. You never stop learning in this business. This is a perfect example of a vehicle coming in with multiple issues. Since there was no history with this car and the car came in as a no start you have to stay on your toes for sure.