Random hickup while driving, sometimes throws code 42

Asked by Jan 15, 2017 at 09:04 AM about the 1991 Chevrolet C/K 1500 Silverado LB 4WD

Question type: Maintenance & Repair

So i work about 45 minutes from home. I have a 1991
k1500, 5.7 efi. Lately at least one time during my trip to
and from work, while crusing in 4th gear at around 50
mph, the tach will bounce and it feels like the truck
hickups. I have code 42 in the trouble codes, anyone
know if this is possibly related?
The worst time it acted up, the rpms would literally drop to
zero and the truck would stall unless i kept my foot on the
gas... ended up bump starting it in second gear to make
my turn. It died again at the very next stop sign, so i shut
the key off and restarted it. It was a little slow to crank
over but once it started again the engine light went away
and it drove just fine all over again..
I dont know if its related to an oil pressure switch, I
Had the oil pressure drop problem, swapped the oil
pressure switch out, and still had a problem, so yesterday
i hooked up a mechanical gauge and found out that i have
40 psi cold and about 18-19 psi at operating temp. Before
i unplugged the oil oressure switch, the truck would idle
down at stop signs to around 500. Now it stays closer to
1000 rpm.. but still hiccups on me during a drive and after
it warms up.. and usually only huccups once per trip.. any
ideas?! This is driving me crazy!! Thanks in advance!!!

1 Answer


CODE 42.... Trouble Code 42 indicates that there may be a malfunction in the Electronic Spark Timing (EST) system. During cranking, the timing is controlled by the ignition module and the ECM grounds the EST line. It expects to see no activity on this line at this time. When the ECM enters EST mode, it applies +5 volts to the BYPASS line and expects to see voltage variations on the EST line. Reasons for the ECM to set Code 42: System in BYPASS mode (i.e. EST line supposedly grounded) but activity sensed on EST line. System in EST mode (i.e. BYPASS line driven with +5 volts) but no activity seen on the EST line.... Possible causes include: 1. BYPASS line is open or grounded 2. EST line is open or grounded 3. Poor connections between ignition module and ECM 4. Poor routing of EST harness and/or poor quality ignition wires (EMI induced electrical noise 5. Faulty or incorrect ignition module 6. Faulty ECM

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