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Misfiring and Backfiring #743209
November 26th 2018 8:24 pm
November 26th 2018 8:24 pm
Joined: Nov 2018
Posts: 3
OR
8
83GMCVandura Offline OP
stranger
83GMCVandura  Offline OP
stranger
8
Joined: Nov 2018
Posts: 3
OR
Hi,
New to the group and with a new to me van. Love the van and really want to become a real vanner! I've done my own car maintenance for a long long time and thought that buying an old van and fixing it up would be fun.

My prize is a 1983 GMC Vandura G2500 that has a Starcraft conversion done with running boards, fender flairs and some custom interior. The van was used as a Band van when it was new going from gig to gig. The guy I bought it from used it for trips from Oregon to Mexico. Though it had been sitting in a barn for the 9 years before I got it, it's got some miles odometer reads 3262 miles, but I'm sure it just turned over 200K. It's had an engine swap, out with the stock 305 in with a Mr Goodwrench 350 crate engine. From the skimpy records I got with the vehicle I figure the engine has between 50K and 70K miles.

I've had it for a year and a half but haven't got to do much with it. Life can be busy. When I got it I needed to take it through DEQ here in Oregon. It didn't pass. I had the entire exhaust system replaced and a new catalytic converter installed, tuned it with new spark plugs distributor cap, rotor, and air filter and it passed. Since it didn't seem to have as much pep as it should I found a mechanic specializing in carborated vehicles and had him look it over. The mechanic put it on his scope and saw that it didn't have much timing advance. He replaced the vacuum advance unit and put weaker springs in the distributor to allow even more advance. Felt peppier and smoother after that. Drove it on a short road trip and it died. Had it towed to another mechanic they replaced the ignition module in the distributor. Ran just fine on the one and a half hour drive home. When I got home I removed the distributor cap to see the new ignition module, yep there was a new fresh one in there. After my inspection, I started having a new kind of trouble. It would start to miss and backfire after being run for about 15 to 30 minutes. Park it for a half hour to an hour then it would run just fine. Went to another mechanic he said the sparkplug wires had been routed incorrectly and were crossfiring. I asked about replacing the plug wires, and he said he thought there was no reason to. And so the van was "fixed" and Life moved on. It sat through the winter and summer no time to do van stuff.

Now it is time to go through DEQ again it hasn't been used in 6 months, again. So I start driving it around to take it in to be checked seems okay, took it to be tested and it failed. CO is not allowed to be above 200 and it is in the 400 range. After racing the engine it once went back down to 201 on idle, close but still a fail. So I decided that I would replace all the rotting vacuum lines and replace those spark plug wires. I very carefully made sure that the spark plug wires were not touching each other or any thing metal. Really runs great! so smooth and powerful till it gets good and warmed up after 15 to 30 minutes then it's back to misfiring and backfiring. This is driving me crazy! It sound like the timing is way off like it has slipped a tooth on the timing chain, but when it cools off it's just as smooth and nice as can be.

Everything in the ignition is new. New coil, Dist Cap, rotor, spark plugs, spark plug wires. I'm confounded and looking for ideas.

Got any ideas? I'm all ears.

Attached Files
GMCvan285x196.jpg (46 downloads)

1983 GMC Vandura G2500 Starcraft conversion
Mr. Goodwrench 350
200K mi.
AdSense long
Re: Misfiring and Backfiring [Re: 83GMCVandura] #743210
November 26th 2018 9:09 pm
November 26th 2018 9:09 pm
Joined: Jun 2010
Posts: 13,881
Van Diego California
frscke1 Offline
frscke1  Offline

Joined: Jun 2010
Posts: 13,881
Van Diego California
Welcome to the site 83 .... Ill check my '93 service manual ... Ill get back ... it is a TBI right.


My guess is a O2 censor on the exhaust ... computer is getting bad info from it.

Last edited by frscke1; November 26th 2018 9:10 pm.

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ADRENALIN BY THE GALLON & CHASIN RACIN
ONE FOR THE DIRT & ONE FOR THE STREETS
'93 CHEVY G30 454 4X4 SPORTVAN EXT 146" WB
'92 CHEVY G30 454 BEAUVILLE EXT 146" WB
Re: Misfiring and Backfiring [Re: frscke1] #743212
November 26th 2018 9:38 pm
November 26th 2018 9:38 pm
Joined: Nov 2018
Posts: 3
OR
8
83GMCVandura Offline OP
stranger
83GMCVandura  Offline OP
stranger
8
Joined: Nov 2018
Posts: 3
OR
Frskce1

Thanks for your response. The '83 chevy 350 motor is carbureted.


1983 GMC Vandura G2500 Starcraft conversion
Mr. Goodwrench 350
200K mi.
Re: Misfiring and Backfiring [Re: 83GMCVandura] #743222
November 27th 2018 8:27 am
November 27th 2018 8:27 am
Joined: Jan 2014
Posts: 314
Staunton, VA
kursed Offline
enthusiast
kursed  Offline
enthusiast
Joined: Jan 2014
Posts: 314
Staunton, VA
Welcome

Re: Misfiring and Backfiring [Re: 83GMCVandura] #743242
November 27th 2018 1:05 pm
November 27th 2018 1:05 pm
Joined: Nov 2013
Posts: 2,942
Michigan
Dyno_Dave Offline
veteran
Dyno_Dave  Offline
veteran
Joined: Nov 2013
Posts: 2,942
Michigan
Welcome aboard!

[Linked Image]

Is '83 a computer feedback carb? Not sure what year the truck switched over.

Re: Misfiring and Backfiring [Re: 83GMCVandura] #743243
November 27th 2018 1:16 pm
November 27th 2018 1:16 pm
Joined: Nov 2013
Posts: 1,045
West Texas
W
Wedgy Offline
old hand
Wedgy  Offline
old hand
W
Joined: Nov 2013
Posts: 1,045
West Texas
Hey 83, Welcome to the site!

First off, I'd verify initial timing, in spite of the experts... Then,

I'd wager to say, sitting for 9 years, might be crap in the tank, lines, filters, and/or carb. Or. Weak FP, malfunctioning choke, or even vaporlock. Circa, early '80's, I drove a 1 ton C30 with a carbed 350, It would Vaporlock when delivering out in the Desert. Cool off, Fine.
I'd be curious to see what surprises the fuel bowl may hold, at the very least. Good wenching!

Re: Misfiring and Backfiring [Re: 83GMCVandura] #743249
November 27th 2018 8:51 pm
November 27th 2018 8:51 pm
Joined: Sep 2005
Posts: 8,543
Georgia, USA
CatFish Offline
carpal tunnel
CatFish  Offline
carpal tunnel
Joined: Sep 2005
Posts: 8,543
Georgia, USA
With the ignition system in good working order and properly adjusted.
Runs well cool but doesn't like warmed up? High CO as well.
My money's on running rich. Uncontrolled fuel mixture.
Goodwrench swap engines are great but don't usually come with any kind of carb so....
IS the carb really for the 350 of did the PO get drill happy with the old 305 carb and cause a mess?

Thinking out loud here....

Last edited by CatFish; November 27th 2018 8:53 pm.


If you owe the bank $100 that's your problem. If you owe the bank $100 million that's the bank's problem.
Re: Misfiring and Backfiring [Re: 83GMCVandura] #743263
November 28th 2018 1:32 am
November 28th 2018 1:32 am
Joined: Nov 2018
Posts: 3
OR
8
83GMCVandura Offline OP
stranger
83GMCVandura  Offline OP
stranger
8
Joined: Nov 2018
Posts: 3
OR
Thanks everyone for your ideas.

I hadn't mentioned that before I bough the van the previous owner had a mobile mechanic come and get the engine to start and run. Apparently after nine years on a barn it wouldn't start or run. The carb was clean when I got it so I presume it was rebuilt or if not rebuilt than just cleaned out. Also in my endeavors to check out all possibilities, I'd done a flow test on the fuel pump, it was okay, and replaced the fuel filter in the carburetor. The old one was pretty clean, not much debris in the chamber either.

Also looking back at my failed DEQ records I see that I miss stated the fail. It was the HC (Hydro Carbons) threshold that failed not the CO. And, the threshold was 220 not 200. Last time the vehicle passed the HC level was 450 on 1st idle test, then after running the engine at 2500 RPM for 30 to 40 seconds a second idle test showed HC at 175.

Could very well be the old carb from the 305 is feeding fuel to the newer 350 engine. I don't really have a way of knowing if it is the original carb or not, but over the last year when I have had it running well, it runs just smooth as can be. Seems like the carb is working okay. When it goes into its missing and backfiring mode the change is abrupt, like flicking a switch. Doesn't seem like a carb can do that.

I did look at the timing. It was set to about 12 degrees BTC with vacuum advanced to distributor disconnected and plugged. vehicle idling in park. I changed the timing to 6 degrees BTC knowing that will help the HC numbers a little, and knowing that the mechanic who put weaker springs in the distributor was trying to get more advance because I had told him that the van seemed like it was a bit sluggish at the time. . . . And that was the timing setting for my '67 ford mustang I had in collage years and years ago. I've since found some advice that says the timing should be set with trans in drive with the e-brake on.

So I did some more sleuthing yesterday. Even though I had installed a new dist cap and rotor about 500 miles ago, I found some carbon on the rotor from the spring loaded center pin on the distributor cap. Cleaned that up. Also cleaned the ground connection for the coil. It was a new coil, and there was still some paint where the wire connected to ground the laminated core. Checked and re-seated the low voltage wires to the distributor. This morning I drove over to the DEQ for testing again. 10 mile drive on the freeway. All my missing and backfiring episodes had occured while driving city streets around the neighborhood.
Seemed okay on the freeway, though feel good that the freeway had good shoulders to pull off an wait for things to cool if I had problems. Arrived to find a long line out into the street. Took 50 minutes of idling to work through the line. The van never missed or backfired on the trip over or while waiting in line. Finally got to the testing booth and passed. Yea !!! HC was 714 on first idle test, but dropped to 213 on second idle test after the run up to 2500 RPM for 30 seconds. Drove the van around for another hour and no problems.

Still not convinced that the problem won't creep up again but in the mean time I'm happy that I have the new DEQ certification and can keep the registration current and continue to drive and test.

Thanks again for the responses. It's comforting to know there is a supportive community out there.


1983 GMC Vandura G2500 Starcraft conversion
Mr. Goodwrench 350
200K mi.
Re: Misfiring and Backfiring [Re: 83GMCVandura] #743264
November 28th 2018 3:30 am
November 28th 2018 3:30 am
Joined: Sep 2005
Posts: 8,543
Georgia, USA
CatFish Offline
carpal tunnel
CatFish  Offline
carpal tunnel
Joined: Sep 2005
Posts: 8,543
Georgia, USA
High HC or high NOX is the opposite problem. My 94 runs slightly lean with the better flowing exhaust. I always had borderline success with passing emissions and would usually drop the timing to test and then reset it afterwards.



If you owe the bank $100 that's your problem. If you owe the bank $100 million that's the bank's problem.

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