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Joined: Oct 2003
Posts: 9
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OP
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Joined: Oct 2003
Posts: 9 |
this car has been so much trouble, like its possessed. it won't start, have fuel pressure up to engine, have spark, but fuel injectors not firing. checked wiring and ohm test fuel injector cuiruit, checked for good ground for pcm. was wondering what would keep the pcm from firing the injectors. have also replaced pcm, driving me crazy. car is 95 contour se 2.5l atx, o2 sensors new, engine wire harness changed and many other componets in the last year, even changed all the relays. so if someone has any idea why the injector are not firing, please let me know. thanks
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Joined: Jan 2003
Posts: 2,017
Hard-core CEG\'er
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Hard-core CEG\'er
Joined: Jan 2003
Posts: 2,017 |
crank sensor and/or cam sensor. Many have stated that the crank sensor causes the car not to start. 
1996 2.5LT V6 Contour ATX My Tour+help/info pics
E0 rims, Gutted Pre-Cats,CTA Custom Exhaust & Intake, Diablo Chip.
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Joined: Aug 2004
Posts: 94
CEG\'er
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CEG\'er
Joined: Aug 2004
Posts: 94 |
Originally posted by linky:
was wondering what would keep the pcm from firing the injectors.
I'm no expert, but my understanding is that the PCM figures out where the pistons are by looking at the crank sensor. So, for example, when the #1 piston approaches top dead center the PCM gets a pulse from the crank sensor. BUT, the PCM is smart enough that it knows that when #1 is at top dead center, it could be on the compression stroke, OR, it could be on the exhaust stoke. To figure out which stroke its on, it looks for a pulse from the CAM sensor. When the PCM gets a cam sensor pulse, the PCM can very quickly figure out which cylinder should be on the intake stroke, and fires that cylinder's injector. And since the PCM knows the firing order of the engine, it can then figure out which injector to fire next, and next, etc. (It doesn't need the cam sensor any more - just needs it to get started. If'n it doesn't get a cam sensor signal when starting the engine, it can't figure out which injector to fire.) 
The PCM uses the crank sensor to keep track of engine rpm and where the pistons are so it knows when to fire which coil. (It doesn't need to know if the piston is on the compression stroke or exhaust stroke, since the Contour uses a 'waste spark' ignition system - it fires the sparkplug on both the compression stroke AND on the exhaust (waste spark) stroke.) 
Pick up a 'spark testor' ($2) at your favorite auto parts store and check for a spark. If you don't get a spark, then I'd suspect the crank sensor (or its wiring) is not letting the PCM know where the pistons are. If you get a spark, I'd suspect the cam sensor (or its wiring) is not letting the PCM know where the valves are. (If you have a 2-channel oscilloscope, you could check for both the crank and cam sensor signals while someone cranks the engine.) 
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Joined: Sep 2002
Posts: 1,788
Hard-core CEG\'er
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Hard-core CEG\'er
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If you have spark (properly timed), the crank sensor is fine.
The PCM uses the cam sensor to control injector timing - that is, to synchronize the injectors with the spark for each cylinder.
To check for a faulty cam sensor, just disconnect it. The PCM will detect this and will "guess" the injector timing. It will have a one-of-three chance on any attempt of being correct. Usually within three attempts, the engine will start. If it guessed wrong, you will get bucking or backfiring.
If there is still no evidence of firing and you have positively confirmed proper spark, remove the hose from the throttle body and give it a shot of starting fluid while cranking. If it then fires, you know you're not getting fuel through the injectors.
All six injectors share a common power source through the PCM power relay. This same power line also runs to the VSS, IMRC, IAC, MAF, ICM, and more. If your CEL comes on during bulb prove-out, this power line is good to the PCM. If you can get to the injector connector plugs, it would be helpful if you can verify that power is present there.
If the engine started only with the starting fluid, you have power to the injectors, and disconnecting the cam sensor made no difference, I would suspect a faulty PCM or perhaps a short to ground on the Vref (+5V) power line (check that at the TPS).
Steve
98 Contour SE Sport 2.5 Duratec ATX The wifey's car
89 Taurus SHO - 246K miles
94 SHO ATX - 190K
1997 F-150 5.4L ATX - The Workhorse 150K.
ANY THREAD WITH "OMG" or "WTF" ETC IN THE TITLE WILL BE IGNORED!
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Joined: Nov 2002
Posts: 216
CEG\'er
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CEG\'er
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Posts: 216 |
Here's a bit more info on this topic. The cam sensor is used for injector timing, like pdqford said. However, if the pcm doesn't see a cam signal, it'll go into a "bank-fire" mode and hit more than one injector at a time. Also, a short to ground at the v-ref circuit will cause the CEL to stay dark at koeo. If the cel illuminates at koeo, and goes out during cranking, the pcm is recieving a crank signal. Also, the presence of spark confirms crank signal. The injectors are powered from a common source and the pcm supplies the ground. Check for power at the injectors (like somone else said). If present, check for a "break signal" (intermittant ground) on the pcm side of the injector connector during cranking. Could be a bad pcm, wiring problem, or even 6 bad injectors.
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