94 Bravada-Running rich

[Ben91RS]

I'm working on a 94 Bravada (4.3L CPI, auto tranny) for a friend of mine. It has 2 current problems that I am having trouble diagnosing, one is that the motor shakes noticably at idle and hunts for idle speed, almost like it's misfiring, but once you get the truck moving the shake seems to stop. The other problem is that it is running extremely rich.

I did a basic tune-up on it (spark plugs, spark plug wires, distributor cap and rotor, PCV valve, air filter, fuel filter), and the old spark plugs were straight black. After the tune-up, I drove the truck around, and notice the car hunted for idle speed when sitting at a stop sign, and even cut off on me once at a stop sign. After it warmed up and got in closed loop, the hunting idle speed stopped. But, the oxygen sensor gives a reading of 700-900 mV at all times when in closed loop, meaning it's running extremely rich all the time and the motor is not constantly switching from lean to rich like it should. Block learn and integrator values were both below 128 at all times as well. All the other sensors that I checked while driving the truck seemed to be giving normal values (CTS, IAC, MAP, IAT, etc.). I've considered replacing the O2 sensor, since all that unburnt fuel has probably fouled it, but I'd rather fix the rich condition before replacing it.

The check engine light is not on and there are no codes stored.

I checked fuel pressure before driving the truck, and it seemed fine. 60 PSI when turning the key, and somewhere between 55-60 PSI (can't remember exactly) with the truck just idling in park. How fast is that pressure supposed to bleed off? Once the fuel pump cut off 2 seconds after turning the key on, the fuel pressure went down pretty quickly, and was down to 0 in about 5 seconds.

After doing some searching on these forums it seems like it's probably the CPI "spider" assembly or the fuel pressure regulator. But, before I pop the intake off and take a look, does anyone have any suggestions? I'm open to anything at the moment.

[Ben91RS]

Well, I took the top of the intake off, and sure enough, it looks like the CPI assembly is leaking. There are puddles of gasoline next to the 3 injectors on the passenger side. So, I'm replacing the whole CPI assembly, the "nut kit" that everyone suggests replacing at the same time, the upper intake manifold gasket, and the oxygen sensor since the one in their now is probably fouled.

[Chris Stewart]

Sorry for the lateness, good job locating the leaky internals.
The oil's probably pretty funky now.

[Ben91RS]

Well, I'm back, and the Bravada is running rough again.

I replaced the CPI unit and "nut kit" in July, like I mentioned, but recently the new CPI unit (GP Sorensen unit from Advance Auto) went bad. The fuel pressure regulator diaphragm went bad and was dumping raw fuel on the passenger side of the motor. I replaced the CPI unit again last week since there is a 1 year warranty on the part, but the truck is still running like it did before.

When it's first started up, it sputters and shakes. When I'm on the gas, it seems fine, but at idle speed it sputters once again and sometimes stalls. When it stalls and shuts off, it takes a LONG time to get it restarted. The computer is only spitting out 1 code, code 32 for EGR. When I replaced the CPI the second time, I removed the EGR valve and cleaned out the carbon, but the code still pops up after only a couple of minutes of driving. The EGR valve was actually replaced almost 2 years ago according to the owner.

As far as I can tell, the new CPI unit is not leaking, but the O2 sensor is still reporting that the engine is running rich. It switches from about 700 to 200 constantly when warm, but it stays rich a LOT longer than it stays lean.


Does anyone have any suggestions or advice? I'm pretty much stumped right now.

[drdd]

did you clean IAC and MAF ?

Originally Posted by Ben91RS

Well, I'm back, and the Bravada is running rough again.

I replaced the CPI unit and "nut kit" in July, like I mentioned, but recently the new CPI unit (GP Sorensen unit from Advance Auto) went bad. The fuel pressure regulator diaphragm went bad and was dumping raw fuel on the passenger side of the motor. I replaced the CPI unit again last week since there is a 1 year warranty on the part, but the truck is still running like it did before.

When it's first started up, it sputters and shakes. When I'm on the gas, it seems fine, but at idle speed it sputters once again and sometimes stalls. When it stalls and shuts off, it takes a LONG time to get it restarted. The computer is only spitting out 1 code, code 32 for EGR. When I replaced the CPI the second time, I removed the EGR valve and cleaned out the carbon, but the code still pops up after only a couple of minutes of driving. The EGR valve was actually replaced almost 2 years ago according to the owner.

As far as I can tell, the new CPI unit is not leaking, but the O2 sensor is still reporting that the engine is running rich. It switches from about 700 to 200 constantly when warm, but it stays rich a LOT longer than it stays lean.


Does anyone have any suggestions or advice? I'm pretty much stumped right now.

[Chris Stewart]

Sometimes the EGR valve will have a film buildup on the stem of the plug valve and shows as the plug valve sticking open. Working it open and shut finally loosens it up.
What do you see on the spark plugs now? You can grab #3 or #4 easily through the inner fender rubbers behind the front tires.

[Ben91RS]

did you clean IAC and MAF ?

No MAF on this car, but I did clean out the IAC, as well as some of the other sensors, like the IAT. I did take off the MAP sensor and noticed some thick, oily build-up in the MAP sensor hole on the intake and the MAP sensor port itself, and I cleaned that out as well.

Sometimes the EGR valve will have a film buildup on the stem of the plug valve and shows as the plug valve sticking open. Working it open and shut finally loosens it up.

I think this is what happened. I removed the EGR valve again last night, and sure enough a piece of carbon had wedged itself in between the pintle and the hole, causing it not to be able to close off completely. I noticed this when I replaced the CPI unit last week, but didn't think much of it at the time, but I did clean it out then. Now that another piece is lodged there, it looks like it's time to buy an EGR valve gasket with a screen.

What do you see on the spark plugs now?

I have not check the condition of any of the spark plugs yet, but I will shortly.

[drdd]

did you reset the computer by disconnecting the neg battery cable for at least 10 minutes?

I've heard you can 'blow out' the intake by leaving the EGR out of the hole and giving the engine a few revs to blow out the carbon ...

what about swapping that O2 sensor with another one further down the exhaust? or, better yet, replace it.

you didn't use Bosch plugs did you?

all that running rich ... is your cat shot yet?

Originally Posted by Ben91RS

No MAF on this car, but I did clean out the IAC, as well as some of the other sensors, like the IAT. I did take off the MAP sensor and noticed some thick, oily build-up in the MAP sensor hole on the intake and the MAP sensor port itself, and I cleaned that out as well.


I think this is what happened. I removed the EGR valve again last night, and sure enough a piece of carbon had wedged itself in between the pintle and the hole, causing it not to be able to close off completely. I noticed this when I replaced the CPI unit last week, but didn't think much of it at the time, but I did clean it out then. Now that another piece is lodged there, it looks like it's time to buy an EGR valve gasket with a screen.


I have not check the condition of any of the spark plugs yet, but I will shortly.

[Ben91RS]

Originally Posted by drdd

did you reset the computer by disconnecting the neg battery cable for at least 10 minutes?

I've heard you can 'blow out' the intake by leaving the EGR out of the hole and giving the engine a few revs to blow out the carbon ...

what about swapping that O2 sensor with another one further down the exhaust? or, better yet, replace it.

you didn't use Bosch plugs did you?

all that running rich ... is your cat shot yet?

Yep, I reset the computer after pretty much any change or repair.

I've heard of doing that as well, but have not tried that yet. I'm going to try it once the engine's up and running again (still checking some things). The EGR passages in the intake are pretty clean already because I cleaned them out when the top of the intake was off for replacing the CPI unit, but it can't hurt. Especially considering it didn't take long at all for another piece of carbon to wedge itself in the EGR valve after I cleaned it anyway.

The O2 sensor is fairly new, I replaced it in June after fixing the first bad CPI unit, I figured all that running rich probably fouled it out. If the spark plugs don't look too bad from the current problem, I probably won't even worry about the O2 sensor.

And no, I don't use Bosch plugs. If it's a GM car, I stick with AC Delcos.


No, the truck isn't shot yet, LOL. It's not mine though, just helping out a friend.

[drdd]

if another EGR doesn't help the "searching for idle" problem you described, what about the TPS? You got one? If so, a broken or loose TPS can cause intermittent bursts of fuel and an unstable idle cuz the PCM thinks the throttle is moving.

u cleaned the carbon off the butterfly right?

Originally Posted by Ben91RS

Yep, I reset the computer after pretty much any change or repair.

I've heard of doing that as well, but have not tried that yet. I'm going to try it once the engine's up and running again (still checking some things). The EGR passages in the intake are pretty clean already because I cleaned them out when the top of the intake was off for replacing the CPI unit, but it can't hurt. Especially considering it didn't take long at all for another piece of carbon to wedge itself in the EGR valve after I cleaned it anyway.

The O2 sensor is fairly new, I replaced it in June after fixing the first bad CPI unit, I figured all that running rich probably fouled it out. If the spark plugs don't look too bad from the current problem, I probably won't even worry about the O2 sensor.

And no, I don't use Bosch plugs. If it's a GM car, I stick with AC Delcos.


No, the truck isn't shot yet, LOL. It's not mine though, just helping out a friend.

[Ben91RS]

Originally Posted by drdd

u cleaned the carbon off the butterfly right?

I did.


Anyway, I finally fixed it. It did turn out to be the EGR valve. Like I said earlier, another carbon chunk caused the pintle to be stuck open. I cleaned out the EGR valve and passages, and even started to motor up without the EGR valve just to blow out any remaining deposits. I got a new EGR valve gasket (it has a small screen on it to help prevent problems like this), installed it and the now clean EGR valve, and started it up, and success! The engine is running great now and is acting just like it should.

Thanks once again for the help everyone.