Carb engine or EFI (injected)CALIF car or FED,low alitude or high, all 6 answers are on hood tag not sharedthe best books beside GMA real (FSM factory service manual is here) 10x less than the $225 EST. (DREAM)
https://eautorepair.net/Marketing/Default.asp THE EFI HAS NO ADJUStMENT FOR BASE TIMING, it has crank sensor.SO is carb right?ok,I will log in for you, and read it and paste it here, (most cars to do this a timing freeze jumper is needed if any computer ,ecu,ecm,pcm used.)the CARB mess. this 5L has TVS vacuum device too for vacuum advance that can fail,. a real tech makes sure all that works before any timing change. all that means vac, advance and centrifugal..Delco® High Energy Ignition (HEI) System that connects to a ECM (puter)firing order: 1-8-4-3-6-5-7-2ANSWERING FOR FED CAR ONLY.The ECM controls distributor spark advance (timing) with the Electronic Spark Timing (EST) system. AND DWELL. will now quote the manual more, on key points. this one means I can not see under your hood."The standard High Energy Ignition (HEI) system has a modified distributor module which is used in conjunction with the EST system. The module has seven terminals instead of the four used without EST. Two different terminal arrangements !!! are used, depending upon the distributor used!!! with a particular engine application.To properly control ignition/combustion timing, the ECM needs to know the following information: Crankshaft position.( this determines base time) Engine speed (rpm). Engine load (vacuum). Atmospheric (barometric) pressure. Engine temperature. Transmission gear position (certain models)NEXT system:The EST system consists of the distributor module, ECM and its connecting wires. The distributor has four wires from the HEI module connected to a four terminal connector, which mates with a four wire connector from the ECM.These circuits perform the following functions: Distributor reference at terminal B - This provides the ECM with rpm and crankshaft position information. Reference ground at terminal D - This wire is grounded in the distributor and makes sure the ground circuit has no voltage drop, which could affect performance. If this circuit is open, it could cause poor performance. By-pass at terminal C - At approximately 400 rpm, the ECM applies 5 volts to this circuit to switch the spark timing control from the HEI module to the ECM. An open or grounded bypass circuit will set a Code 42 and the engine will run at base timing, plus a small amount of advance built into the HEI module. EST at terminal A - This triggers the HEI module. The ECM does not know what the actual timing is, but it does know when it gets its reference signal. It then advances or retards the spark timing from that point. Therefore, if the base timing is set incorrectly, the entire spark curve will be incorrect.An open circuit in the EST circuit will set a Code 42 and cause the engine to run on the HEI module timing. This will cause poor performance and poor fuel economy. A ground may set a Code 42, but the engine will not run.The ECM uses information from the VAC and coolant sensors, in addition to rpm, in order to calculate spark advance as follows: -High VAC sensor output voltage would require MORE spark advance. -Cold engine would require MORE spark advance. -Low VAC sensor output voltage would require LESS spark advance. -Hot engine would require LESS spark advance.end quote , if spark is retarded we check this first, if bad BINGO for retard hot. engine must be hot 180f+ what you want is static timing adjustment (dynamic is the job of ECM)TRAPPED VACUUM SPARKThe TVS and related parts do retard spark cold engine and if TVS fail hot, then it will be retardedeven 4 degree's as you have.maybe your mech is wiser and knows the TVS is bad and that parts is expensive .The TVS and evil friends (VAC) are super hard to source now, as many say, and me. 38years old. see the timing adjustment jumper on some 84(5L) timing freeze. seem my page link how.EFI:the crank sensor is the device that determines spark starting time, then the ECM does the rest and sure carbb TVS.and this off youtube , comments"distributor delay valve, the parts stores have a very hard time finding it."the TVS theory"This results in a constant spark advance. if working, seems to me it is not but I can not watch his timing light do wrong. not ever. and see , if coolant is 180f to 200, not 150f. see?but if spark time is same hot and cold engine, TVS path is dead. as a door nail.and rare parts will never be cheap.If true their $ quote is bogus and x10 common non 38 year old cars;. very few left on the road.so rare now many state do no smog check them at all. now. (less cars than smoggy boats)
TESTING
See Figure 2
Do It Yourself Diagnosis and Repair
