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Discussion Starter · #1 ·
I have a C3500 with a dump body I use for my business occasionally.

Truck is rough and an electrical nightmare, has a draw and you can't leave it sit without the battery disconnected overnight.

What's known:

It had "no codes" and ran fine when I bought it in November last year and its gradually gone "downhill".

It had a MAF code P0101, 102, or 103 and poor performance and misfire code. I replaced the MAF with a Cardone MAF, blew out the air filter, double checked for leaks on the intake (several loose boots) and it did not help.

It ran okay just with a code all winter and now for summer its misfiring progressively badly and harshly hard enough to bang the driveshaft cruising at 55, and now having trouble just backing around the driveway to move it to mow.

I'm thinking

Intake gaskets (but I don't have coolant burning/intrusion)

The most obvious one is the one on the top of the intake with the thumbscrew to the snorkel, you can literally jiggle it around, I feel like it shouldn't move that much but can't find that particular gasket that seals it.

The ignition hasn't been touched. Not sure where to start.

I don't know the current codes because I have to disconnect the battery.... But assuming MAF/misfire
 

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On my 1/2 tons of that vintage, the EGR would stick open and cause that. Not sure if the 1 tons have an EGR valve. #1 symptom was a 1-2 shift hard enough to bark the tires, even at 1/4 throttle. And, they never threw a code.

Also, I never, ever, had good luck with aftermarket cap & rotor. They had to be ACDelco. Always reuse the original screws, or you risk breaking the screw hole(s) open.

And, that vintage also was prone to getting a hairline crack in the distributor reluctor, which has no wires, yet would cause misfiring randomly. The replacement distributors never seemed to have that issue.

As for the mystery draw, several of my '98 1500, 2500, and 3500 trucks had that symptom too. We traced it out to the dimmer switch for the dash lamps. When working correctly, the interior lamps would "theater dim" to off after exiting, and it was the theater dim part that failed. That's only if your switch looks like this...
Font Electronic device Circle Electronics accessory Audio equipment
 

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Discussion Starter · #3 ·
Font Auto part Circle Drawing Diagram

Im assuming I need to replace this entire part (#16) air intake adapter as theres no seal offered to replace. The TB is full of dirt so I'm assuming a post MAF intake leak is likely.

Unless I just goob it up with grease or silicone and pray.

Product Jaw Font Gesture Screenshot

All the ignition parts look really new and it does start and idle ok.

Hey McCall thats good to know. i'll need to figure out what fuse supplies that little beast and disconnect it/do an amp draw test on it. The draw is SUPER annoying.
 

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#21 seals the end at the throttle body, and #14 seals the other.
#16 is just hard plastic.
 
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#21
Eye Gesture Font Circle Illustration
15713884
Is still available thru other (non-GM) vendors.
 
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As for the EGR valve, stuck open (which is what would happen to mine) can be tested by inserting a homemade blocking plate, or simply blow air in the dirty hole once removed.
 
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Discussion Starter · #7 ·
#21 View attachment 651190 15713884
Is still available thru other (non-GM) vendors.
This is the rubber boot in between the resonator the adapter.

The adapter itself has rubber that seals and those are stretched and damaged. Sort of like an axle seal.

I applied some softer silicone to it (gasket dressing) we will see if that helps.
 
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The resonator can be blocked off, and will not change the way it runs. The engine will simply have a different sound under acceleration.
 
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Discussion Starter · #9 · (Edited)
Gauge Speedometer Measuring instrument Font Circle

Asphalt Font Publication Book Logo

I siliconed the suspect joint shut and took it on a test ride. Drove crappy as usual.

No signal at all from the MAF sensor getting to the PCM the needle never moves. Will have to check the FSM to see which wires should have power, etc.

I tore the dash apart and pulled the connector out of the headlight switch and the truck started a day later, which is so far promising. Thanks heymccall. What was the solution for that switch? A different year switch? Aftermarket? OEM?
 

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Discussion Starter · #10 ·
So I tried to use the FSM posted here but it's barely searchable (most annoyingly) and missing critical circuit data for specific sensors. I just know I need power and ground at two pins. Of the connector. Be nice to know where the other wire goes. I might try later years than 1997 and see if there is more info, but it seems like these come up extremely short for advanced diagnostics unless the gas engine stuff is straight up pulled out on purpose.
 

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The headlamp switch we just replaced with a part from Napa or GM dealer.
 
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Discussion Starter · #13 ·
The problem ended up being covered by the FSM.

I got 5 v from the yellow 5v ref wire.

The meter showed ground pink power wire. So I started chasing wiring gremlins. But when I got back to the fuse box, I found the ENG-1 (for the power wire) circuit with a blown 30a fuse in it. So I replaced it with the appropriate 20a fuse.

Got 12v immediately and problem solved. Was thrown off by ground on hot wire. This circuit covers nearly a dozen components so over all of those it must somehow have found ground.... The fuse is holding and continued to hold through an engine degrease and test drive. These circuits were covered by the FSM provided on the forum.

Typical diagnostics start with the fuse for a reason, I guess.

Need to work more on the draw problem, going to do an amp test with/without the headlight switch to confirm.

Thanks heymccall!!!
 

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Discussion Starter · #14 ·
So the recurring blowing of this fuse is ENG-1 is pretty common. Eng 1 is 12v hot for many of the emissions components under the hood. EGR, Evap, MAF, Cam sensor, o2 sensors.

Basically any of the bad components on this circuit, when commanded to operate by the ECU, will then short to ground and blow the fuse. Once the system gets into open loop and starts activating things like the 02 sensors, the fuse blows. So it COULD also be the EGR valve sticking and overloading the circuit. Going to try to disconnect the o2's and drive it and see what happens before buying them but they aren't too expensive.

Truck is actually a joy to drive without the malfunction.
 

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Discussion Starter · #15 ·
With all the components except the MAF and the VCM backfeed disconnected, it still blows the fuse once it gets to open loop. You can feel the truck buck then hesiteate. Maf signal goes away when fuse blows.

Going to try the old maf as a last ditch. If it still blows its the VCM. I may have a spare somewhere.
Font Line Slope Parallel Rectangle
 

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With all the components except the MAF and the VCM backfeed disconnected, it still blows the fuse once it gets to open loop. You can feel the truck buck then hesiteate. Maf signal goes away when fuse blows.

Going to try the old maf as a last ditch. If it still blows its the VCM. I may have a spare somewhere. View attachment 651600
Tried running a new wire on that circuit?
 

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Discussion Starter · #17 ·
I don't believe the wire is bad. The fuse would blow instantly as soon as the key is on, as this is an ignition hot wire. Something changes when it goes into open loop. With everything disconnected, it's either the MAF or the VCM itself is grounding out with some command and a break on the internal board causing it to ground.
 
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Discussion Starter · #18 ·
The nightmare deepens spent an inordinate amount of time matching year, options and even rear end ratio and the VCM won't work won't and has an extra 5 pin connector on it. i cannot find **** on whether the yard sent me the wrong computer. Wasted precious time and money on something so stupid.

The yard most local to me won't even confirm if they have the right year chevy truck in the yard much less if they have blue interior parts which I need.
 

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Discussion Starter · #19 ·
So the VCM did work, the extra 5pin connector is for electronic 4wd.

The yard screwed up as the unit I ordered was off a 2wd truck. There was paint from the original VCM on the connector stuck to the rubber that prevented the connection.

I was able to heat the engine to over 180 and not blow the fuse but many flash related issues persist, fuel gauge and perhaps EGR with a code p1046, and there was an instant awful misfire whole truck shook. When I pulled the egr to inspect for a stuck pintle the gasket got lost instantly.

I ordered a flashed unit for my vin from Flashmasters on ebay and will need an egr gasket hopefully will get to the bottom of this.
 
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Discussion Starter · #20 ·
Vcm did not help. I'm ****ing stumped
 
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