Quick question about a chevy 4.3 TBI.

I bought the engine from the junkyard, its from a 1993 chevy pickup.

I got the harness and ECM from a 1988 Astro van.

Only mismatches between the two is the 1988 4.3 had a vacuum actuated EGR; 1993 had an electric actuated EGR. TPS was a different style plug. Other than that the harness went right on the engine no problems.

I made all the connections etc, changed the plugs, and it fired right up no problems. It idled high, but after I plugged the brake booster vacuum line it idled right down very smoothly.

Ive been driving it around a little bit and it seems when the engine is under load and Im accellerating, as soon as I go over maybe 1/4-1/2 throttle, the engine sputters and backfires out the intake, sometimes with a nice fireball. I let off the throttle and it will ALMOST die, but if I give it a quick jab of the throttle it comes back to life to a nice smooth idle. If I then go to accellerate the vehicle again, if I do it very easilly without hard accelleration it will operate normally, but press the pedal anymore and it backfires. Ill see if I can take a video of it...

Im confused because it idles perfectly....however if I leave it idling, every 1 minute or so it will sputter and start to sound like its going to die, but it will not actually die, just runs roughly. If I quickly jab the throttle it instantly revs up and then settles back down to a nice smooth idle.

If I free rev the engine in park, its perfect. Revs right up, no stumbling, sounds great etc... Its only under load that it does the backfiring.

I dont have a timing light, and I know I should reset the timing (unplug bypass connector, set to 0*), but why should I have to do this? How could the timing have gotten off in the time that the junkyard pulled the engine and I put the harness on it and started it up??

I have the ALDL port wired up and I will try to put the tech 2 on it tomorrow...

any ideas? :help:

thanks
ben

Could possibly be a toasted camshaft, or timing (somehow) is out to lunch. Computer maybe not matching up to current engine specks? Could be a few different things...

if the cam was worn/jumped a tooth wouldnt that mean poor running at idle too?

You need to set/check base timing just to be sure it's on. Who knows if it was off even when in the junk yard truck.

I suspect a loose connection or a vacume leak somewhere, because it is very intermittant.

Don't forget to check grounds.

Any check engine lights or codes?

I need to check for codes. At the very least Ill have a code for no VSS; there is no electronic VSS on the vehicle.

One other thing; I dont have the alternator hooked up. I just charge the battery enough to start it and then test drive it; I suppose theres another potential problem right there! :o:

I dont have a timing light so Ill have to try to track one down... I assume setting timing on it is like any other carb'd engine? Find the mark on the crank pulley, connect to #1 plug wire, and adjust distributer until the pulley mark is at 0* with the reference mark on the block???

I drove it around again today and the problem seems to be intermittant.

It seems to be worst at 40-50% throttle. If I go WOT its not bad, same if I go slowly. But in that midrange area it backfires and chokes. Mostly out of the intake, but some exhaust pops too. When it backfires out of the intake, its either a fireball or a misty cloud (fuel I assume?).

I hooked up the Tech 2 to it and it doesnt show any codes, but that cant be right...I dont have a check engine light hooked up but Ill do that and then check codes "the old way". Bi-directional controls with the Tech 2 work, and I can read out all the data; TPS shows .54 volts at 0%, and I think 4.9x at 100%. It is going into closed loop and at idle, it constantly flip flops between rich and lean, so thats good I guess because it shows the O2 sensor is working and that its not staying rich or lean constantly.

I couldnt drive it too long because it starts to run hot; I dont have any fan on the radiator. I just mess around with it until it gets to 215-220* or so and then shut it off and let it cool.

I still need to set the timing tho.....

One other thing, Im skeptical about my alternator. When I first hooked it up, it charged (14volts at idle), but then after a few minutes it stopped charging. Its a Delco CS alternator; I only hooked up one of the wires, via a resistor to the IGN circuit, to get it charging. I dont see how I could have burned something up, because I did have a resistor there. I will try hooking up the other excite terminal on the alternator and see if that gets it charging.

If the alternator is bad, whats the best to replace it with? The CS or the SI alternator?

ben

Check your plug wires.... should have changed while the engine was out... plugs too

Older big blocks are notorius for backfiring when wires have gone bad, small blocks and the 4.3's will also do this

could also be running too lean.

what would cause it to run lean? Watching it with the Tech 2 it seems to be perfect (alternate quickly between rich and lean)

I will take a snapshot of it with the Tech 2 to see what happends when it backfires. I dont know how accurate it will be tho, because of the slooowwwww data bus.

Check your plug wires.... should have changed while the engine was out... plugs too

Older big blocks are notorius for backfiring when wires have gone bad, small blocks and the 4.3's will also do this

How could I check the plug wires other than replacing them?

I will pull the spark plugs right now to see if theyre black, etc, or any other signs of abnormal burn.

I put new AC Delco plugs in it, but I did not gap them (.0045??)

I know I know, I need to gap them, but I cant see that causing the problem. I really need to first make sure the timing is set correctly, but Im trying to track down a timing light..

Replace the fuel regulator in the throttle body, ive had 10 TBI S-10's ( 8 -4.3's and two 2.8 TBI ) A few of the 4.3 models would hesitate pretty bad and some times backfire. replaceing thie regulator fixed the problem for me, Ive went thru plugs/caps /rotors , even setting the timing and after replacing the reglator all runs well. These regulators get worn and leak off fuel contributing to poor drivabiltiy, hope this helps.

Also when setting timing there is a tan wire with either a black or white stripe that needs to be unplugged before setting base timing. This is the wire that the computer uses to adjust timing . unplug the wire, set timing, then plug the wire back in with the engine off.

yeah I need to get a fuel pressure guage to check the regulator....

how much is a new pressure regulator?

Also...could the EGR cause any of these issues? The engine has an electric EGR solenoid, but the harness was from an older 4.3 that used a vacuum actuated EGR

Its been a while since ive bought a regulator, there not that bad thru Napa I believe. Not quite sure on the EGR. While yor working on your Throttle body, unsrew the Idle Air control valve and give it a good cleaning with some carb cleaner and in the hole where you pulled it from too. These get gummed up and usually give some idleing/drivabilty stumbles if they are dirty. One more question, Do you have a fresh fuel filter installed?

Its been a while since ive bought a regulator, there not that bad thru Napa I believe. Not quite sure on the EGR. While yor working on your Throttle body, unsrew the Idle Air control valve and give it a good cleaning with some carb cleaner and in the hole where you pulled it from too. These get gummed up and usually give some idleing/drivabilty stumbles if they are dirty. One more question, Do you have a fresh fuel filter installed?

yes, fuel filter is brand new and the fuel pump is brand new; its an aftermarket fuel pump, e2000, high pressure but that shouldnt cause a problem. Everyone on Pirate 4x4 runs this pump on their TBI rock buggy's and say it works perfectly.

Fuel pressure should be 15psi

Timing is like the old days AFTER you unplug the timing connector.

Plug wires can be checked by useing an ohm meter. Stick a lead in on one end and the other, wiggle wire while reading the ohm reading. No more than 10k ohm per 1 foot of wire.

Check timeing, check timeing, check timeing

MMMM, building a crawler huh? Cool :cool:

MMMM, building a crawler huh? Cool :cool:

yeah I guess you could call it that :D

Bet ya never seen one of these before. :cool:

http://i28.photobucket.com/albums/c247/penguin1494/DSC_0085Large.jpg
http://i28.photobucket.com/albums/c247/penguin1494/DSC_0091Large.jpg
http://i28.photobucket.com/albums/c247/penguin1494/DSC_0101Large.jpg
http://i28.photobucket.com/albums/c247/penguin1494/DSC_0124Medium.jpg
http://i28.photobucket.com/albums/c247/penguin1494/DSC_0078Medium.jpg

Now Thats cool!! You probably will never see another4x4 bug around. what chassis are you using? again that is Fuggin sweet!

You can use an ohmmeter to check the wires. They usually run about 10K ohms per foot. Whenever I have had a bad one, it always checks completely open, or infinite ohms. It only takes a few minutes to check them, and will likely find the problem. Backfiring is commonly caused by crossfiring, where the spark plug fires while the cylinder is on the intake stroke.

Now Thats cool!! You probably will never see another4x4 bug around. what chassis are you using? again that is Fuggin sweet!

Its a 1976 CJ7 frame that I had to do some stuff to. Keeping the engine in the back and flipping everything around was a good amount of work. Theres more pictures in my garage.

it only weighs about 2600 lbs and has hydroboost brakes...stops on a dime, literally.

I still dont have a timing light, but just for grins I disconnected and reconnected the "set timing" wire (didnt reset the distributer tho). It seems to run better, but still a little funny at some throttle positions. This leads me to beleive its a timing issue; once I get a timing light and actually set the distributer to 0*, im *hoping* it will completely fix it.

i take it your not running a air cleaner when its doing this? if not put one on then diagnose. they like an air cleaner on while driving. tbi is a speed density system. dosent like a open intake

correct, right now Im not running an air cleaner...I will get one though.

Try a throttle body swap if you could find a good used one, maybe this might help elimnate your problem. It sucks when you have to resort to changing parts. The throttle bodies on these engines get worn out( more so the PSI reg. like i said) and cause all sorts of drivabilty issues. Good luck, you'll get it.

if the cam was worn/jumped a tooth wouldnt that mean poor running at idle too?


..not always. But now with added information, it could be a few things that have been mentioned.

..not always. But now with added information, it could be a few things that have been mentioned.

so its a pretty safe bet that the timing is off?

Basics first.

Sounds like it could be a intermitently failing MAP sensor. If its giving faulty readings, or no readings it could cause this same situation. The engine will run without it, but not well...basically as you have described, it will run better at full throttle and idle, but not well through the mid-range.

The TPS may be the culprit...it has 3 ranges and from what you are saying once you get past the middle range it smooths back out. Test it and see...also you mentioned different connector ...did you splice or change out the TPS?

If it was me......I would start off with checking and testing all ignition sources such as wires, plugs, etc.......then move on to find TDC on #1 and mark "0" on the balancer. Then proceed to properly set the timing and get the basics out of the way.

If that doesn't fix it, test fuel pressure (GM's are finicky with what pressure is there...to high runs like crap...to low won't barely run), then move on to make sure all your connections have been pushed all the way on, then double check your grounds and even add a few if possible....(can never have to many or to large of grounds) Make sure you have grounds from battery to body, body to chassis, and most importantly battery to engine. Even go as far as undoing the bolts that are attatched to the grounds and retighten them to make sure they are not corroded or what have you.

Then at that point you can start suspecting sensors and so forth from that point. But you have to start with the basics on something like this or you could end up spending more time and money trying to blindly figure it out and find the issue......though sometimes you can get lucky, it doesn't happen very often.

just my .02 cents....