96 Vortec 305--Rough Idle and Smells Rich

[cris well]

Hi all.

I just bought a 96 Silverado 305 that runs great down the highway but idles rough and spits and sputters a bit at take-off but picks up shorty after with great power and it's not throwing any codes.

I also noticed that it smells really rich like a carb that is loading up at idle.

I changed the fuel filter and I was gonna check the fuel pressure next before I try anything else.

I'm thinking it may be a stopped up converter also. It has a small converter that looks like it may be aftermarket--I haven't looked at it too closely though.

Thanks for any info.

[ChevyTech]

It could have a bad coolant temperature sensor that the computer monitors, which is separate from the sensor for the dash gauge. It is used in determining the mixture.

It could have a leaking fuel pressure regulator causing it to be rich. A leaking regulator will show up in a fuel pressure test. The pressure will drop as soon as the pump stops.

It could also have leaking poppets if it still has the original fuel spider design and has not been upgraded.

[cris well]

Thanks ChevyTech.

I forgot to add that it cranks right off the bat when cold but when it warms up I have to crank it over quite a few times to start it up and sometimes hit the gas too.

Is there a way to check that coolant temp sensor and where is it on the engine?

I'll also rent a fuel pressure tester and check the regulator.

[ChevyTech]

You're welcome!

The sensor is in the front of the intake, right in front of the thermostat.

It can be tested with an ohm meter.

There are test instructions and specifications in the thread.
http://67-72chevytrucks.com/vboard/s...d.php?t=379142

[iluvmy72]

Originally Posted by cris well

Hi all.

I just bought a 96 Silverado 305 that runs great down the highway but idles rough and spits and sputters a bit at take-off but picks up shorty after with great power and it's not throwing any codes.

I also noticed that it smells really rich like a carb that is loading up at idle.

I changed the fuel filter and I was gonna check the fuel pressure next before I try anything else.

I'm thinking it may be a stopped up converter also. It has a small converter that looks like it may be aftermarket--I haven't looked at it too closely though.

Thanks for any info.

When was the last time the injector spider was replaced?

[speedygonzales]

Originally Posted by cris well

I forgot to add that it cranks right off the bat when cold but when it warms up I have to crank it over quite a few times to start it up and sometimes hit the gas too.

Sounds exactly like a ruptured FPR diaphragm.

You should be smelling the oil on the dipstick to see if it smells like fuel.

Another thing you might also try is listening close to the CSFI connector after pressurizing the fuel system to see if you hear fuel spraying out of the FPR. You can turn the key on with engine off KOEO and back off then KOEO a few times to pressurize the system then key off (by someone else) while you listen closely to the connector.

The FPR is located right next to the connector inside the upper intake manifold. Sometimes, but not always it will make a noise as the fuel comes out.

First though you should be checking for fuel contamination in your oil. I'd be willing to bet your FPR is ruptured with a good amount of $$$$

[cris well]

ChevyTech--I changed the coolant temperature sensor and it runs and cranks much better now but still slightly hesitates off-idle and still smells a little rich. I think I'll drive it around a bit before I do anything else. The exhaust system may be full of unburnt fuel or something making it still smell rich.

iluvmy72--Not sure about the injectors, probably never changed. I just got the truck.

speedygonzales--I checked the oil and it doesn't smell like gas. Would it leak enough to make the oil show as overfilled on the stick? If so, I'll keep a check on it to see. I'll also try to listen and see if I can hear gas spraying near the FPR.

[cris well]

To speedygonzales--I send you a PM.

To all others--Can I change the FPR without removing the spider assembly and the poppets from the intake?

I'm scared the plastic pieces are gonna break when I try to get the poppets out.

[speedygonzales]

I did not get a PM

Before you go doing this, you need to check the fuel pressure at prime and after the engine is shut down. If the FPR is bad, the fuel pressure will be low at prime and drop off fast after shutdown. BEWARE, that is not the only cause of fuel pressure dropping after shut down. There is a check ball in the fuel pump sending unit that holds pressure after the engine is shut down. If this check ball is sticking it will drop fast also. And these stick very VERY often.

However, if the check ball is bad, it will cause the engine to have to crank a long time before it fires up if the engine sits for a few days without running. The prime pressure will be ok if only the check ball is bad.

If the FPR is bad, the prime pressure will be low AND pressure will drop off fast after shut down. Be careful to notice the symptoms.

You can change the FPR by only removing the upper plenum. You will need to unplug the large central plug at CSFI unit. You will need to remove the fuel lines that run from the bell housing to CSFI unit. The fuel lines are the biggest PITA.

I don't remember but I think you can leave the upper radiator hose on and still get the upper plenum off. When you do remove it, you will need an upper gasket set to replace the orings you pull apart. It will come with the oring below the throttle body but you do NOT need to remove the TB from the upper plenum.

Of course you will need to remove the throttle cable and cruise control cable. As well as a couple of vacuum lines and PCV hose. And about 10 bolts have to be removed. Oh yeah the coil and ICM have to come off.

[b454rat]

Mine was doing something similar an EGR valve was sticking.

[cris well]

To b454rat--Yes I had read about the EGR valve but I'm not getting any computer codes and I also unplugged it for a while but It didn't change the way it was running and it was still hard to start once it warmed up. I have a micrometer that I rented and I will check it out properly though.

To speedygonzales--Maybe the PM didn't come through. It is showing as a sent message on my end. No worries.

Anyway, I read what you said in the last post and it cranks fine after sitting for a few days or a few hours. Only when it is warmed up does it not want to crank properly. Then idles really rough, runs rich, missing off-idle but runs great down the highway after initial stumble off the line.

When I checked the fuel pressure, It would read about 51-53 psi when switching key on/off and a little more while running and idling. It would jump up higher when I revved on the gas a little. When I turned it off the pressure dropped to zero in less than 15 seconds.

I watched a YouTube video of a guy pulling the old spider assembly out and it broke a few of the poppets off. I was trying to avoid pulling the spider out just to change the FPR.

I was thinking of replacing the spider with the updated version Amazon.com: ACDelco 217-3029 GM Original... if I pulled it out and broke something.

I think you said that I can leave it in to change the FPR? Is that what you mean by TB/throttle body--the spider assembly?

[speedygonzales]

If your pressure is changing when you jump on the throttle, the fuel pressure regulator is working and does not appear to have a hole in it. However, Your pressure being low may be the whole problem. The FPR may be keeping pressure low but doubtful.

What you need to do is pinch off the return line and see if your pressure will go up. If your pressure goes up when you restrict the return line, the pump is ok. If the pressure does not go up, the pump is having a problem.

Big question here. Did you ever change the fuel filter???????

You may have a bad injector(s). You should pull and inspect all the plugs. You should also pull the upper plenum and then reconnect the fuel lines and pressurize the system to see if any injectors are leaking.

You may need a new spider unit. Don't get one from an unknown source. Here is one from Summit. But it is best to pin down the cause of the low pressure first.
http://www.summitracing.com/parts/smp-fj504/overview/
All these units are manufactured by Standard Motor Products no matter what name is on them. Summit is the best retailer there is. Stick with a reputable dealer.

The throttle body is the metal body on top of the upper plenum (almost looks like a carburetor). The tube from the air filter connects to it. And the throttle cable hooks to it and the butterfly plate opens inside it when you press the throttle pedal. It does NOT have be removed from the upper plenum if you don't want to. However if you are going through the trouble to do this job and you will get the o ring for below it, you should change that O ring. And clean the TB real good while it's off.

Getting the old injectors out is not a big deal. When you put the new ones in. Pay close attention to the numbers on the injector body and the bottom plenum. You will see what I'm talking about when you get there. Don't sweat it.

Here's a picture of the throttle body.
http://s1371.photobucket.com/user/St...nx9th.jpg.html

Here's a picture underneath the upper plenum showing the CSFI and injectors. Notice the FPR is the round kinda brown thing in the front connected to the CSFI body.
http://s1371.photobucket.com/user/St...glvrv.jpg.html

[cris well]

Yes the fuel filter was the first thing I replaced. The second was the coolant temperature sensor. The sensor helped it idle a little smoother and stopped some of the off-idle sputtering but not all of it.

I'll try pinching the return line next and check the plugs too.

[cris well]

Now the only way I can get a reading on the FP gauge is to screw it on the schrader valve with it running. After I got the gauge to read again I clamped the return line off.

I got about 65 pounds or more with the truck running for about 15-30 seconds and when I shut it off the pressure still dropped but much slower. It took a few minutes (2 or 3 maybe a little longer) to drop back to zero. I could clamp the hose off tighter and try it again but I'm scared I'll damage the rubber hose if I clamp too tight.

I was also reading through a thread on another site and the guy was having a similar problem with his. Later, after changing the spider assembly, he said his gas gauge started bouncing around so he changed his fuel pump and it fixed his problem.

My gas gauge is jumping around too. Is that a sign the check valve is bad?