94 running rough with a twist

[astromark]

Hi, My 94 astro vin w, cmfi, 125k mi is driving me to drink, more . It is running rough at idle and driving. Shaking like a couple cylinders are dead all of a sudden.

Parts replaced, Full tune up, nut kit, head gaskets, manifold gaskets, ignition module. The spider and egr have been previously replaced. All tune up parts are ac delco. Checked wires several times. Checked egr again, still running rough.

The twist,
I went to set the base timing and the light seemed to skip a beat. I had the timing perfect so I reconnected the timing connector and restarted it. Still running rough and the timing light blinked good then double time then missed. Not steady, a very irregular pattern.

Another odd thing is that in my repair manual my wiring matches a 93 not the 94. The door has 5/94 on it.

All this and no codes. Except for the carbon in the egr before this problem.

I am not a rookie, in fact I think it is easy to pull these motors and rebuild them. On an older astro I owned I did a v8 conversion in a weekend. Maybe this van needs one now.

I hope you guys can help.
Thanks.

[old_master]

Was the distributor cap and rotor also replaced? Were they AC Delco also? What brand of spark plugs and spark plug wires were used? Was silicone dielectric used on all secondary ignition terminals, (cap, rotor, both ends of spark plug wires)? What is the fuel pressure?

Fuel pump operation:
The fuel pump circuits are designed to energize the fuel pump continuously any time the key is in the START position. When the key is in the RUN position the fuel pump will operate provided the engine is running. If the engine stalls, or is not running for any reason, the PCM will shut off the fuel pump after approximately 2 to 3 seconds.

Fuel pressure test:
Install the fuel pressure gauge to the test port near the distributor. Turn the key to the RUN position but do not start the engine. While the fuel pump is running, fuel pressure must be 60-66psi. After approximately 2 to 3 seconds, the fuel pump will shut off. Fuel pressure must remain between 55-60psi for several minutes. A drop in pressure below 55psi indicates a leak. The most likely place is the fuel pressure regulator, or the lines under the upper intake manifold.

[astromark]

Yes to everyting you asked except the fuel pressure. I am picking up a tester on my way home from work today and I will test it this afternoon. I planned on testing it anyway. But I don't see how the fuel pressure would cause the strange spark.

[old_master]

Fuel pressure will not affect spark, I am just curious for diagnostic purposes.

[astromark]

Ok, key on = 62 psi, pump off 58 psi and holds steady. Vehicle started 55 to 58 step on gas 60 to 62 whether in gear or not.

I'm not real good with new vehicles. Give me points or a one wire HEI anyday.

I hope my last response did not seem sarcastic. I am baffled by this and I am not exactly sure what the ecm looks at for spark control. If I had the ecm schematic and the computer program it would be much easier for me.

Thank you for the help.

[old_master]

By the way, welcome to the forum. You're doing fine with the diagnostics, don't let the system intimidate you. These systems are extremely reliant on good fuel pressure and delivery. The fuel pressure is good, and there are no leaks. You mentioned in your first post that the miss developed "all of a sudden". We need to make sure the "basics" are ok. Double check the firing order, 165432 clockwise rotation, #1 cylinder is driver side front, and it's position in the cap is straight ahead. Check for vacuum leaks while the engine is running with carb cleaner. Check all around the intake manifold.
Make sure PCV valve is intact. If you have a vacuum assist power brake booster, pinch off the line and see if that makes a difference in idle quality.
Make sure the air intake tube connections are tight.

I'm not questioning your abilities here, but you have done a substantial amount of work involving several systems and sometimes something simple gets overlooked creating a real head scratcher. Post back your findings. Hope this helps.

[ericn1300]

check the distributor shaft for excessive wear or binding at the upper bearing.

[astromark]

Ok, checked firing order, a ok. Sprayed carb cleaner all around intake, vacuum ports and hoses, no change. Hydraulic assist brakes. PCV new ac delco with tune up trying to fix this problem. Distributor was removed and I checked for free movement/play recently. It was reinstalled in exactly the same location.

While checking vacuum hose connections I removed hose to heater controls and rpm increased but still rough.

Question: Does crank position sensor have to do with spark? I'm thinking if the timing chain jumped it would throw everything off or am I thinking too much.

The van always ran a little rough but this was a sudden change. Very rough while driving. One thing, S*P injector cleaner was added (not by me) before this happened. What should my mileage be with AWD? It gets about 14 mpg.

[astromark]

Update: After phisically checking there is no crank position sensor or cam position sensor. The wire color codes and sensors match a 93 even though mine is a 94.

[ericn1300]

Originally Posted by astromark

Update: After phisically checking there is no crank position sensor or cam position sensor. The wire color codes and sensors match a 93 even though mine is a 94.

not unsual for the engine and body year to be different. on my 86 Safari i had to step to the 87 motor to get the right parts.

[old_master]

I wouldn't worry about the timing chain, they're pretty much bulletproof. Haven't seen one go on a 4.3L yet, and I've serviced a BUNCH of them, some with upwards of 300,000 miles. Not to say it's impossible, but highly unlikely that it jumped time. To check slack in the chain, remove the distributor cap, then with a breaker bar and socket on the crankshaft damper bolt, rotate the crankshaft clockwise until the timing mark is on TDC. Have an assistant hold the distributor rotor by hand, while you rotate the crankshaft counter-clockwise until the rotor just begins to move. Read the degrees of timing chain slack on timing the index. More than 12 degrees slack warrants chain and gear replacement.

Do you know what cylinder(s) are misfiring? With the engine at idle, pull plug wires one at a time and notice idle quality.

[astromark]

Thank you for that body year/engine year difference. I have never come across that before.

My assistant is cooking a big dinner. I wil have to check the timing chain slack another time besides you said it's unlikely that could be it. I did pull the wires one at a time, ouch, Can't find my plug wire pliers. I did it a few times to make sure ( oww oww oww) #2 cylinder has no change. Pulled plug, checked gap, used finger for compression test (tester broken) all ok. I Put the plug on the wire and grounded the base and the spark looked weak to me. Coil problem?

[old_master]

The coil can't differentiate between cylinders, so if it's weak on one cylinder, it will be weak on all cylinders. With #2 dead, it could be a faulty spark plug or plug wire, that would explain the weak spark and the miss. There could also be an intake manifold vacuum leak on the underside which would be undetectable with carb spray. Disconnect the #2 plug wire and idle the engine for 15-20 seconds, shut the engine off and immediately pull #2 plug and check for fuel on the electrode. It should be wet with fuel. If not, I would suspect a vacuum leak or a stuck/restricted poppet valve on #2 cylinder.

[astromark]

Ok, performed test and plug was wet a little. Checked #4 wire with spare plug had same spark. Swapped #2 and #4 wire and pulled #2 off distributor. No change in idle = wire good? Pulled #4 and noticeable change.

There was also a noticeable difference in exhaust manifold temp from #2 and the rest.

[old_master]

Yup, wire good. Did you try another plug in #2 hole?