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1996 blazer V6 4.3 Vortec running rough

1996 blazer V6 4.3 Vortec 212000 miles

Drove about 500 miles this weekend (200 there, 100 running around and 200 back) and the car was running great until I finished the last leg of the interstate driving. When I had to stop at a red light the car was idling very rough. When I put on the gas (after the light turned green) it seemed like the car lacked power in the lower gears. I don’t know if it is missing on a cylinder or what is going on. The plugs and wires were changed in the last 15,000 miles. I did some off-road driving this weekend and some muddy water (the liquid not the blues man) splashed up in to the engine compartment as far as the top of the air filter housing. I can not take it in till tomorrow so I wanted to see if any one had any ideas.

Any ideas?


Check engine light on?

Did you gas up at a generic gas n gulp? Or have a slow gas pump (often, the pump is slow when the tank in the ground is low). Try some dry-gas and injector cleaner, maybe you have some watery sludge gas.

No check engine light.

Funny thing is my milage has been better this trip than last. 1st fill up of the trip was at a “dodge city” truck stop. Second was a Walmart gas station. The fill ups were on Friday and I did not notice any thing untill Sunday. In part the motor is shuddering in place. I wondered if I broke a motor mount or some thing. What brand Gas and Oil treatment would you suggest?

You might want to have the fuel pressure tested. Your engine has the Central Port Fuel Injection system. Under the intake manifold is the CPFI or the spider assembly. This consists of a central fuel injector with a pressure regulator with plastic tubes that run to each cylinder. At the end of these tubes are the poppet valves. These poppet valves utilize the fuel pressure to blow them open so fuel can be injected into the cylinders. If the fuel pump is weak and doesn’t provide the proper fuel pressure, which is 63 PSI, the poppet valves don’t open completely and the engine runs rough.


Makes sense. That really tears it. I will take it to the dealership then.

thanks all.

Don’t take it to the dealer. Any independent shop can check the fuel pressure on your vehicle.


We had a post awhile back from a person that related various driveability symptons. Well I did my best and explained things that needed to be done and even provided information related to PCM reprograming (from a manufactures TSB. Well the OP wrote back that the problem was caused was 3 worn out spark plugs, since then I only get involved in cases that fit my parameters of “problem cases”.

I want (well what I really want is for no one to have any type problem with their car) is for the OP (that means you) to state how current mainteniance is and to say “I have opened the hood and have performed a very good visual inspection and I just cannot see what the problem could be”

Dealer or Indepentant does not matter, just be sure that the mechanic is a driveability expert and not a mechanic who’s usual duties do not let him become very well experienced in the methods used to diagnois and repair driveability concerns.

Oil changed, plugs and wires less than 15,000 miles old, under the hood the motor is shaking in it’s mounts (maybe due to how rough it is idling) and the plugs are under enough stuff that I worry about getting them back in if I managed to get one out. Belts look good. I have been told that (after a service a few months ago) they got it running smoother but a V6 is a bit off balance and a 200,000 mile v6 can only be expected to be so smooth. I keep the oil changed and the maintenance done on it as budget permits. New front breaks but I doubt that would be involved. This might be tangentially related but my AC will not switch from bi-level to vent/heat/defrost. The temp door is working so I can make it hot or cold. I have checked and do have vacuum at the vacuum ball (enough? I do not know). My budget has not allowed me to take it in to get that AC issue checked since I have had good vacuum every were else I was not worried about it. Now if this rough running is a Vacuum issue it might be related.

Guess I could have asked before but I will now, Is this a sequencial multi port injected Blazer (meaning does it have 6 fuel injectors with wires to them or is this some form of CPI (central port injection?) My memory says that the Blazer went to multi-port a few years before the full size truckx. Knowing what type of injection used will be very helpful when you go to interpet fuel pressuree readings and there are quite a bit more problems with CPI (that the one with the spyder and is pretty sensitive to poor quality fuel according to GM). Notice I said “quality” and not octane, they are not the same. You could look on you RPO code list (I believe on you glove box door) and look for the code,in fact the 8th digit of the VIN will tell you engine type (we know it is the 4.3) but also injection type. You can give the Dealer Parts dept a call for the code if you don’t know.

I will look tomorrow.

Looks like bad gas just pulled ahead in the polls. Last night it was shaking/rattling and/or rolling as before. This morning I get in to take it to the shop and smooth as it all ways had been. After a few min of driving I get a miss fire here and there but nothing like yesterday. I think that my last fill up at a less then pristine Kangaroo gas station might have been the cause. After that I drove at least 65-75 mph the entire way until I got off the interstate. It might have happened sooner if I had not been going. I do not want to have to pay to have my tank drained and cleaned but will if I need to. What is the thoughts one that. I have around 1/2 tank. Should I run it about all the way out or put in the additives now?

Don’t put too much faith at all in the “bad gas” explaination unless there are others that got gas at the same place/time and everyone has a sample.

Gotcha, Not putting faith in it per say but this is my line of logic such as it is tell me what you think. I am computer security guy and I know that hooking up some ones random Dogma will get you run over by Karma lol

If a plug/wire/rotor contact goes bad it often times stays bad.
If it goes from good to that thing they mix paint with at lowes and then back to mostly good I tend to think it might not be mechanical. Could some thing have gotten in to the distributor cap and is working it’s way out? Really I am not more than a change my own oil guy and open to any thing from the wise fruit of knowledge to best guesses. My plan is to let it run out and then fill up with gas from a known good source and hope for the best.

Let me know your injection type, I can offer better help.

Sorry I forgot, got caught up at work. Looks like it has a Central Sequential Fuel Injection (CSFI) or a Sequential Central Fuel Injection (SCFI) and one site calls it a Sequential Central Port Fuel Injection (SCPFI). Man some people love to make things complex.
I am trying some gas drying agent from Advanced, I hope that if it works cool but I do not think it can do any harm or make thing worse.