1994 Sierra 1500 4.3 TBI "Z" 5 Speed

[mproctor83]

Hello, everyone. Been a long time reader of this great forum as well as the other Chevy truck forums (fullsizechevy.com, silveradosierra.com) , but this is my first time posting. A lot of knowledge here so I am sure someone has some idea what is going on with my truck.

I have a '94 GMC Sierra 1500 2wd 4.3 5 speed. The truck has been running fine until the other day I went out in the morning to start it up and it would not stay running without applying throttle and barely has enough power to move. I have to rev the heck out of it and let the clutch out very slow just to get it going. I have replaced the fuel filter and tested fuel pressure (12-13) psi. Also, I put a spark tester on each wire between the wire and plug and get spark on all cylinders. Once I get it moving, if I give it more than half throttle, it starts surging (i think that's the word for it, it feels almost like it's losing all power for a second or two then comes back for a second or two then back to no power. Kind of like the key is being turned off and on repeatedly or like going from full throttle to no throttle) between 1500-3000 RPM. Above 3000 RPM (when I can actually get it the RPMs that high in gear) the roughness goes away slightly but it's still very noticeable and feels like I'm dragging a house and the tach jumps all over. I can get the engine to rev up all the way to redline when it's in neutral or with the clutch in, but it is still very rough. I had a friend come over and was playing with the vac lines while I was in the cab giving it enough throttle so it would stay running. After he took the lines off and reconnected them it ran slightly better for a while until I shut it off. When I started it back up it's every bit as bad as it was before. The check engine light is not on while running, but it does come on when the key is first turned to on so I know the bulb is good. If anyone has any idea what could be causing this issue I'd be very grateful if you could shed some light on the situation for me. If you need any more info from me let me know. I am home all day right now since I can't work without my truck, so I will be glued to the forum. I live less than 1/4 mile from Autozone so I can rent tools if need be. I have had this truck for around 7 years and never given me any trouble until this. I'm really puzzled because I drove it home one night and it was fine, then when I started it the next morning it was like this. I have searched and tried some of the suggestions (fuel filter, check fuel pressure, air filter, spark check), but I don't see anyone with the same symptoms as I have. Sorry in advance if this has been answered and I missed something. I also posted this over at fullsizechevy.com and silveradosierra.com as I am desperate for help. Thank you for reading,


Mike

 

[RichLo]

What did the check engine code say?

and welcome BTW.

 

[michael hurd]

Surging can be caused from lean conditions. ( not enough fuel )

It sounds like you might have multiple issues if the tach is jumping.

Distributor pickup coils leads in older vehicles can be problematic, often the insulation on the wires leading to the connector can be cracked, and the wire fatigued enough to give intermittent connections.

Also, the reluctor magnet can have many small cracks in the magnet, this leads to mutliple flux lines, giving the computer false information when it comes to accurate positioning.

Standard Motor Products DG44 for the distributor shaft and reluctor. ( includes new distributor rotor as well )

A thorough examination of the pickup coil and the magnetic material in the reluctor ( may need a magnifying glass ) will help you diagnose these.

Alternately, instead of tossing parts at the stock distributor housing, you could elect to replace the entire unit. ( Just as inexpensive and faster )

 

[mproctor83]

Thanks for the quick responses!

RichLo, The only code that comes up is 12 which I believe just means the ecu is getting power and functioning.

michael hurd, would this Duralast Gold distributor from Autozone (http://www.autozone.com/external-en...-new-distributor/194364_0_4618/?checkfit=true) be ok, or would it really be worth it to get one from the Chevy dealer?

Also, the tach only jumps when in gear and trying to accelerate under hard throttle. When in neutral it moves smoothly throughout the range of 1000 to 5000

 

[michael hurd]

I can't tell you what the durability of the Duralast unit is going to be like compared to the OE unit. It does appear to have brass terminals on the cap and rotor, which is a better deal than the aluminum ones that are typical of cheap units, even the factory replacements.

As long as the pickup coil and module are quality units, it should give you no more trouble than an OE unit.

I had posted a link to an article about a reluctor magnet being cracked some time ago. ( was a V8 distributor, but the same applies to the V6 )

Best to pull the cap and rotor, examine the pickup and the pole piece carefully with a lot of light and a magnifying glass before tossing parts and money at this.

 

[J_lope82]

Did you check the fuel pressure in idle or while under load(driving)?

 

[tanman_2006]

I am going through this on my 94 350. Check the fuel psi under load and check the TPS.

I bet one or both crapped the bed on you.

 

[skylark]

I do agree that it sounds like an ignition issue but I have one other thought. It may have jumped time. This usually happens when the engine is shut off and the crank is still trying to spin. Line up the balancer with the timing mark and pull the distributor cap. if it isn't pointing at #1 or exactly 180 out then it has most likely jumped time.

 

[mproctor83]

Thanks for the ideas, everyone. Ok, so a few more bits of info. I took the cap and rotor off yesterday to inspect and noticed the rotor was cracked where it connects to the distributor shaft and all the metal parts looked very worn so I replaced the rotor and the cap. The dist shaft/gear seem ok (only about 1/16th inch of rotational play). Also, since I already had it apart, I took the ignition control module to Autozone and had them test it. The machine showed the ICM was bad so I picked up a new one and put it on (used the included grease on the underside of the ICM). Put the new cap and rotor on as well. After replacing the parts I disconnected the brown wire under the dash, set the timing and replaced the wire. I took off the EGR and cleaned it thoroughly with a small wire brush and brake cleaner. It seemed to be working fine, I could move the diaphragm by sucking on the vac line and it looked as though it was fully closed and would seal well. The truck still wont idle when cold and dies as soon as I put it in gear and let the clutch out a little bit. When warmed up, it will just barely idle and still wants to die when accelerating even when if hold the RPMs high and let the clutch out very slow. In neutral, I am able to rev the engine pretty easily (up to about half throttle, if I go much past half throttle the engine stumbles up to around 3500 RPM) even when cold and it doesn't seem to be missing as bad as before.

When I checked the fuel pressure it was at idle. I don't know how to check it while driving(maybe I needed a different type of fuel pressure gauge kit). I set the timing back at zero with a timing light after replacing the cap, rotor and ICM. I looked for the article about the reluctor and pickup with no luck. I did find some other articles with google and tried to check it as best I could but maybe you could tell me exactly what to look for. The sensor and magnet looked pretty clean. The white plastic around the magnets was a little discolored/yellowed but overall looked ok with no cracks. The plastic cover over the magnets turned slightly more than the shaft of the distributor, about 1/4" of rotational movement(not sure if this matters). I checked over the ground wires from engine to body/frame and they are clean and tight. Will try testing the TPS in the morning.

 

[michael hurd]

Here's the link to the article:

http://forums.hotrod.com/hot-rod/70/7429233/pit-stop/gm-tbi-no-pulse-while-cranking/