Since I fixed the misfire problem my 1998 C5 convertible became gradually harder to start. When it started it ran just fine.
I replaced the battery but it didn't help and now it doesn't start at all.
Fuel pressure is OK and the injector balance test checks out OK. Spark tested OK with spark test lamp.
CKP reads when I crank and the CMP reads when it starts. Should the CMP give readings also when cranking?
I have the GM Tech II.
Any ideas would be highly appreciated.

 

I have never tried that method but the CMP (Cam sensor) is not needed to start the engine but you WILL need the crank sensor to be working. You should see something while cranking with the CMP...A bad CMP will cause those longer starts but the car will eventually start. With the Tech 2 check the RPM while cranking...if it reads around 150 RPM or so you are getting a crank signal to the PCM...I'd also check the TPS 1 and 2 voltages KOEO (key on engine off)....TPS 1 should be around 1.04 and TPS 2 should be around 3.96...if the PCM thinks the gas pedal is pushed down to beyond 85% throttle angle the injectors are shut off and the car will not start...this is like the "clear flood crank" when you hold the gas pedal to the floor and the car only cranks...did you try spraying starter fluid or Brake Kleen into either the brake booster or PCV line...this will rule out a fuel delivery issue. You can pull off an injector connector and with a test light connected to battery POSITIVE probe the wire that is not pink while cranking...the test light will blink if you are getting injector pulse !!

 

Thanks for answering, C5 Diag.
I went out again to do the tests you suggested. And of course the engine started directly, twice.
Then I started to crank again, again, again and again and I got this DTC


While I tried and tried to start it suddenly started and after awhile the MAP DTC turned to passed.

Have I found the problem?. Faulty MAP or bad contact?

 

A P0410 should not affect the engine starting !!…P0410 is not the MAP sensor.

 

Terribly sorry. I posted the wrong screenshot. here is the MAP one.

 

MAP voltage should be high with key on engine off…somewhere between 4.8 to 4.9 volts and 101 Kpa…at idle it should read around 1.35 volts or so and 35 Kpa…is this what you see
??…if that high voltage stays high when the engine is started that will set the DTC !!..P0108 is “circuit high input” !!…the MAP sensor is not pulling the 5 volt reference voltage to ground when starting !!…have you had the intake manifold off ??

 

Thanks a lot!
I will check this tomorrow and post the results

 

When I tried today it started twice without problem.
I checked the values before starting, key on engine off…somewhere between 4.8 to 4.9 volts and 101 Kpa
Idling, a little bit hight values than you stated 1.4 volts or so and 40 Kpa.
After the two starts it didn't start anymore and I monitored the MAP values while cranking.
They showed just a little bit less than the KOEO values, say 4.8 volts and 99 Kpa. What should it be you think?
And yes, I removed the intake manifold half a year ago.
Today I disconnected the MAP connector and tightned the "connectors" in the connector to make sure there isn't bad contact issue.
Then it started again twice and I took the car for half an hour drive without a problem.
Came home and switch off. Then it didn't start again even though there were a few attempts and some backfires.
What do you figure?

 

When it’s not starting and just cranks see if you are getting an RPM signal with the Tech !!

 

yes, I do. It varies 100-150 RPM.
I also got another P0108.
I wonder what the MAP should read during crank no start.

 

Between key on and during a crank the MAP voltage shouldn’t change because there is still not a lot of vacuum in the intake manifold…just to show you I have a picture below and I also graphed the MAP voltage during a crank…no change !!….try spraying something like starter fluid or Brake Kleen into the intake manifold somewhere and see if it attempts to start…you have to rule out fuel delivery !!…do you have fuel pressure key on ??…just for the heck of it I would see if you are getting injector pulse (test light connected to battery positive probing the striped injector wire with it unplugged while cranking)

…I myself would not rely on just scan data !!

 

I didn't manage to start it with starter fluid. Maybe I could figure out where to inject it properly.
I also failed with the test lamp. it only worked across the battery.
I used a scope connected to battery neg and the non- pink wire if an injector.
When cranking there are only irregular random pulses and occasionally a small burst which I can correlate to the engine firing a few cylinders.

I have my pressure guage connected to the fuel rail. It has unfortunately no bleed valve and I don't know what error that will introduce.
My friend observed a pattern when we tried to start many times.
First, when switchiing on the ignition the fuel pump primes instantanously to 60 PSI.
When the fuel pump stops it drops instantly to 50 PSI.
Then the presure rises slowly over 1-2 minutes to 55-56 PSI. If I wait for this and crank, it will start. Almost always.

This is the closest I been to something reproducing the problem.
What should happen after the priming finsh? Should it drop to 50 PSI like that, or should it stay at 60?

 

You can inject it into the brake booster hose or the PCV…what do you mean by “I also failed with the test lamp” ??…do you mean it’s not working or you failed the injector pulse test ??…like I had said connect the test light to battery POSITIVE…remove an injector connector and probe the striped wire while cranking…the test light will blink if the PCM is grounding the injector circuit….it’s as easy as that !!…are you using an oscilloscope??…when you turn the key on the static fuel pressure should not drop more than 5 psi for 1 minute after reaching the max pressure…the pump will only operate for 2 seconds then shut off so you will see the pressure decrease then…there are a few things that can cause this….leaky injector or feed pipe check valve…if the pressure bleeds down quickly you will have those hard starting issues…you may have to turn the key on and off a few times to get that pressure up sufficiently to start the car !!

 

I have injected starter fluid in the brake booster and PCV but there is no reaction.

Injector pulse test passed when I did it in the garage. Last time I tried outside in broad daylight and didn't see the faint pulses.

During priming (KOEO) the pressure is 60 PSI, dropping instantly to 50-51 PSI when fuel pump stops.
See attached video.
Is this normal behavior?

 

Problem solved. Don't buy aftermarket sensors.
During my earler misfire problem (which turned out to be a cracked #7 spark plug),
I replaced several sensors, crank, cam and MAP-sensors.
After the misfire was gone I got into a period of hard to start. The engine cranked and cranked
without starting and after many tries it finally started and then run fine.
This went on for several weeks.
Searhing the forum here and other sites I found that a bad cam sensor can couse a hard-to-start
but since I had put in a brand new cam sensor I found it hard to believe that this could be the problem.
Anyway, I removed the passenger front wheel and opened up for access to the PCM and back.probed
the crank and cam sensor inputs with an oscilloscope while the engine was running fine.

Making sense of the waveforms. Difficult to find any refernce on what they should look like.
Maybe some one could post them as a refernce?
Afterquite some time monotoring the signals I could see the the cam signal became irregular in frequency and pulse width.
Pulled the intake manifold and replaced the cam sensor with the old original one and then the engine started every time.
Problem solved I thought and took it for a ride. After a couple of miles it died on me. Now the engine started but died after a seconf or so,
every time I tried to start. Luckely I had my Tech2 in the car and it showed a MAF sensor failure.
I found out that when resetting the error the engine started again. Starting to drive back home it died again twice but I finally made it back home.
Searching the web again and reading the service manual for MAF sensor problems I decided to follow the trouble-shooting procedure

in the service manual. To my surprise it started wiith a check of the MAP sensor. It says,"use the TECH2 to check that the MAP signal is within the range 0.4 to 4 volts". Mine showed 0.79 Volts.
I have also read somewhere that you can start and run the engine if you disconnect the MAF sensor. With the engine running I disconnected the MAF sensor and the engine died immefiatley. I left it discoonected and the engine would start.
Finally, I replaced the brand new MAP sensor with the old original one and it started again.
I now managed to drive in to town for a long overdue carwach and still made it home again. It seems to work now and hopefully I have managed to fix the proböem
I read somwhere that these cars need OEM sensors and you should not replace them with aftermarket cheap ebay sensors.
It seems that this is somthing to be aware of.
I hope this post can help someone in the same situation.

 

Cam/Crank from my 2001…as you can see on the 24X crank waveform the signal varies because of the notch widths and spacing in the reluctor wheel…the 1X cam rising and falling edges should be in this location relative to the crank signal.