Jim Grant's Tech Tips
‘97 Chevy Astro, No Injector Pulse
Q: We have a ‘97 Chevy Astro van in the shop that wouldn’t start.† We got spark, fuel pressure was good, but the scan tool was showing no injector pulse during cranking.† I disconnected the harness to the injectors and tested with a noid light and the light flashed on for all 6 injectors.† Later I tried to do an injector imbalance test with the scan tool because after I pulled the plugs, #4 was very black and the rest were tan.† I was running out of ideas and did this just for the heck of it.† The first try I got 3 injectors to pulse and the other 3 didn’t.† The next try none would pulse.† One more thing I might note:† when we tried using starting fluid it would backfire through the intake system.† I know sometimes this can indicate cam timing being off so I did a compression check and got around 125 psi on all cylinders so timing should be good.† I'm at a loss.† At my previous employer we had a similar problem with a Chevy Blazer with a 4.3l Vortec engine and it wound up being a bad computer.† Our shop was not the one that did the diagnosing to find out it was a bad computer so I don’t know what procedure they used.† I was wondering if this might be a possibility with the Astro and if so how would I test for a bad computer?
- James Rosato
A: It is always a little on the nerving side when it gets down to a possible computer.† After all, once the replacement is plugged in you own it whether or not it fixed the problem.† To add to the fun, you not only buy a computer, you have to get it programmed for that vehicle and once thatís done you own it whether or not you plug the computer in.† Be sure that there are no trouble codes or pending codes in the computer.† If there are you need to check their possible relationship to the no start condition.† Check the power supplies (fuses) and the grounds for the computer.† More than one fuse is used to power the computer and a bad ground will cause the computer to go nutty.† Is there a security tell tail light on the instrument panel?† If there is and the light is on, the computer may be commanding a no start condition.† If it is activated, the computer will only provide brief injector pulses.† The engine may act for a moment like it wants to start but injector pulse will be lost or become erratic.† The scan tool has a menu to view antitheft operation.† Also, watch out for aftermarket alarm systems.† Theyíve turned out to be a big pain in the backside for causing misleading diagnosis.† With the weird stuff out of the way, you have to start with the basics.† For the computer to operate correctly it needs good information.† Check the crankshaft position sensor for a clean and steady signal.† The computer uses this information to locate crankshaft position and speed of rotation to provide spark and fuel delivery.† During cranking you should see ignition spark and injection pulse in phase with each other.† If the spark is erratic you need to look closely at the crankshaft position sensor.† If the spark is clean and matching engine rpm but the injectors are not then check the signal from the Throttle Position Sensor (TPS).† A failing TPS can trigger the computerís program to activate what is called the clear flood mode.† This command is activated if the TPS voltage is high.† The computer in response will reduce injector pulses or even turn them off while cranking the engine.† There are 2 other critical inputs that the computer needs and that is the coolant temperature and load sensor information.† Both of these can be checked with a scan tool while checking† TPS values.† When diagnosing a possible computer failure you have to confirm that the computer is receiving “all” the information it needs to operate (good inputs).† If the in going information is good, but what is coming out is bad you could be looking at a replacement computer.