TBI timing help…

So I have a 1987 Sierra Classic with the 305 TBI. Last week my son called me and told me it wouldn’t start at school. I went and verified no spark. Replaced the distributor as well as the ignition coil. Got motor to TDC, unplugged the ECM bypass and set base timing to 0 degrees. Unplugged the battery to reset the ECM, plugged the bypass wire back in and fired it up. With the ECM plugged in, it runs like crap. When I unplug it, it definitely runs better and doesn’t stall.

Tonight I set the base timing at 6 degrees BTDC. This made things infinitely worse. Could I have a bad ECM?

mstumpf said:

So I have a 1987 Sierra Classic with the 305 TBI. Last week my son called me and told me it wouldn’t start at school. I went and verified no spark. Replaced the distributor as well as the ignition coil. Got motor to TDC, unplugged the ECM bypass and set base timing to 0 degrees. Unplugged the battery to reset the ECM, plugged the bypass wire back in and fired it up. With the ECM plugged in, it runs like crap. When I unplug it, it definitely runs better and doesn’t stall.

Tonight I set the base timing at 6 degrees BTDC. This made things infinitely worse. Could I have a bad ECM?

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The ECM is usually the last thing to eat itself, not impossible but would save that for after sorting out a lot of other things 1st.

Did the new distributor include a new module as well? If not that would be the top of my suspect list.

When you set the motor at TDC, did you pull a plug and verify that its really TDC? Well worn 40 year old crankshaft timing marks have been known to slip on the damper pully. Your timing light might say 0 but the piston might not really be at 0. I like to use something wood (a chopstick from the local chinese place is a good choice) to follow the piston while rotating the engine with a wrench and feel when it reaches the top - a screwdriver works but I like using something that won't scratch the piston or cylinder wall if it slips.

Are you getting any ECU error codes? I used to get a #42 ESC error which is usually a bad grounding between the distributor and body, a secondary ground lead to the firewall cleared that up.

mstumpf said:

Replaced the distributor as well as the ignition coil. Got motor to TDC, unplugged the ECM bypass and set base timing to 0 degrees.

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1. The ICM was probably what was bad, not the entire distributor
2. It isn't clear how you set the timing after you dropped the new distributor in. Timing should be set with a timing light on the #1 wire on a warm engine (if possible) with engine running and EST bypass unplugged.

Before I bought and replaced the whole distributor, I bought and replaced the ICM which didn’t help the problem because I was still getting no spark.

I pulled the #1 plug and set it to top dead center by using a wooden dowel rod and watching it rise.

The truck runs fine with the EST bypass wire unplugged, but when I plug it back in, it wants to die whenever I come to a stop.

I am also getting the famous “code 42” when the bypass is plugged back in.

mstumpf said:

Before I bought and replaced the whole distributor, I bought and replaced the ICM which didn’t help the problem because I was still getting no spark.

I pulled the #1 plug and set it to top dead center by using a wooden dowel rod and watching it rise.

The truck runs fine with the EST bypass wire unplugged, but when I plug it back in, it wants to die whenever I come to a stop.

I am also getting the famous “code 42” when the bypass is plugged back in.

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OK these become clues (or "data points" to the more tehnically engineeringly inclined). Code 42 is the not very helpful "electronic spark control" error - lots of things can trigger it. Disconnecting the bypass to set the timing will set it, so you need to clear the codes by disconnecting the battery for 10-15 mins to get a meaningful reading.

Mine happened because of bad grounds between the distributor and firewall - I added an extra ground run from one of the coil mounting bolts/studs to the firewall to rectify it when I got hit with it.

Other things that can cause it are bad plug wires near the multi-pin connections on the distributor, or good plug wires but too close to the multipin connections, either of which can induce false signals to the module, and confuse the system.

Pull the glovebox lid off and check all the connectors going into your ECU which is cleverly hidden back there, a loose connector there could play havoc.

I Recently changed a distributor on a 1995 4.3 s10 , symptoms were that it died taking off out of there driveway and no start. After. Not even after a week did it start or do anything. Cranked it over checking spark at coil wire. I actually got a blue spark. I figured maybe bad cap or rotor. Checked and there almost new looking and the distributor looked to be from 1995. Confirmed good fuel pressure so I replaced the distributor and set it close to where it was. Fired up instantly and ran surging a couple minutes and was smoothly running 10 minutes later. Set timing by ear warm around 600-700 rpm and made sure it didn't ping under load because I couldn't find the little tan wire lol. Went back later and set to 0°. I'm guessing it was maybe getting spark to the plugs but the fuel injectors were not getting pulse.
The theory on the bad balancer is a good one.
Maybe set it by ear and see how it runs and or just find true TDC and see what you're balancer shows to rule that out.

Ok I checked the ECM and connections, all looked good. No corrosion and all securely fastened. Check the wire routing to make sure none were near the ICM wiring.

Seems to be running ok but as soon as I put it in gear the RPMs drop and it wants to stall if it doesn’t stall. I did just notice that it randomly sounded like it revved up for a second and then stalled minute later. Also hearing an awful sound after it stalled until I disconnected the bypass. Wtf can that be?? (Edit to add that was the injectors running after motor stalled)

Guess I’ll pull the #1 plug again and get to TDC and see what the balancer says again

mstumpf said:

Ok I checked the ECM and connections, all looked good. No corrosion and all securely fastened. Check the wire routing to make sure none were near the ICM wiring.

Seems to be running ok but as soon as I put it in gear the RPMs drop and it wants to stall if it doesn’t stall. I did just notice that it randomly sounded like it revved up for a second and then stalled minute later. Also hearing an awful sound after it stalled until I disconnected the bypass. Wtf can that be?? (Edit to add that was the injectors running after motor stalled)

Guess I’ll pull the #1 plug again and get to TDC and see what the balancer says again

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Hold the phone! Injectors were running after the engine stalled? As in trickling fuel, or full spray? That could be a symptom of a different problem. My (probably incomplete - others feel free to fill in what I miss) comprehension of how the injectors work is the computer gets a pulse from the ignition module which tells it to trigger them in sync with the coil providing spark. And that triggering happens on the ground side - the injectors always have +12 running to them and the system grounds the other side of the injector to make it spray. The duration of the spray is calculated by the computer using the throttle position, manifold pressure, temp sensor and other reading. If the injectors were spraying after stall-out, that sounds like they were shorted to ground on the signal side.

In theory if something was inducing a voltage on the pulse line from the module to the computer it could fool the ECU into firing the injectors - or your initial feeling that the ECU was failing could turn out to be right. Those injectors should not trigger unless the engine is turning. And disconnecting the bypass for the spark control stopped them?

Coolant temp sensor? Maybe unplug it and see how it acts

Yeah the injectors kept running once it stalled and the key was still in the on position. I thought that was weird too.

Ok I tried unplugging the coolant sensor but it didn’t change anything.

So I uninstalled the distributor, found TDC again, reinstalled, made sure plugs and wires were in their own happy places, made sure the bypass was unplugged, retimed, but still wanting to stall when shifting into gear.

When its idling out of gear will it rev and idle good.
Maybe IAC or tps malfunctioning?

Two questions... the first one is really dumb, so please forgive me:

Are you for sure putting the timing light on cyl 1, rather than cyl 2? Cylinder #1 is left bank (driver side)

What does the injectors' spray pattern look like when it's running? Is it a nice, fine, cone pattern?

The injectors should not continue to spray after stalling. I might recommend a cheap oscilloscope so you can see the injector trigger and start diagnosing that.

Swap ign. module out of the new distributor.

I still can't tell how the timing is being set. if you pulled the distributor, put the engine at TDC, then you need to stab the distributor with the rotor pointing as close to cylinder 1 as possible. then start it and let it warm up. then unplug the EST bypass and put the timing light on wire for no 1 cylinder and set timing to 0 deg BTDC. turn off the engine. reconnect the EST and you should be good, or at least at a good place to trouble shoot another issue.