sugarsn10

iTrader: (2)

I've been try to figure this out for a couple of months now. I started a thread about it before, but it eventually died and the problem still persists. I'm hoping maybe some other people have an idea. The car will start right up when it's cold and run great. It will rev up and hold rpms while it is cold. The problem occurs when it warms up. I think it happens when it goes into closed loop. It starts idling real rough and will barely idle, I have to pump the gas to keep it going. Also, it will rev up, but when you just hold the gas it starts to bog down and will eventually die. I got an o2 reading, but I'm not totally sure if the sensor was at it's operating temperature. I've read it should be bouncing back and forth around .45 v, the readings I got varied from 0.03-.08 which is extremely low. It smells very rich at first start up when it is running good, but not when it warms up and starts running bad so I'm guessing it could be running too lean?

Background on what has been fixed or replaced: I bought a rebuilt cc Quadrajet in January since the old one definitely was toast. I put a new cap, rotor, coil, plugs, and wires. I have replaced the o2 sensor, CTS, and the absolute pressure sensor. I had tested all of these and knew they weren't functioning. I ran all new vacuum lines yet vacuum seems a little low. I even tried plugging all vacuum ports except the brake booster fitting and I got about 9 in of vacuum. There is a little bit of moisture around the intake/head mounting surface. Could a leaky intake man. gasket cause this vacuum leak? Originally a tube went from the passenger side valve cover to the air filter housing, but the previous owner put an aftermarket air filter housing that didn't have a spot for it. This is for crankcase venting right? I don't have it hooked up since I'm working on it without the air filter. This shouldn't cause a vacuum leak would it? Thanks in advance for any suggestions.

five7kid

iTrader: (14)

A leaking intake manifold gasket can cause a vacuum leak. It is more typically exhibited as a coolant leak, but it could be vacuum as well - including from the lifter valley (not necessarily from the exterior of the manifold/engine).

The PCV hose you're referring to provides filtered air to the crankcase, as drawn in by the PCV valve. If you have a PCV valve on the driver side, it shouldn't be "leaking" any more than it should, even if you have the hose on the passenger side open. You can get a push-on cap that has the filter media in it to replace the hose that originally went to the air cleaner.

Damon

Is the choke getting full open (choke plate vertical) when the engine warms up?

Also, crack open the gas cap and try again. Just a check to make sure the venting system is working properly and isn't starving the engine for fuel due to vacuum building up in the tank.

sugarsn10

iTrader: (2)

Thanks for confirming my thoughts on the vent. The choke flap does get nearly fully open. Cracking open the gas cap is a good idea, I'll try it out today. Thanks for the advice guys!

sugarsn10

iTrader: (2)

I tried running it with the gas cap open and fully off, but it didn't make a difference. I tried running it multiple times, but it would still bog and not hold rpms when you gave it gas nor hold idle once it had warmed up a bit. I kept just giving it quick shots of throttle for a while and kept it running for about 10 min finally. By this time it was at operating temp of about 220 and it was running great. It would idle just fine and and would hold rpms if I gave it gas.

I figured I could finally now fine tune the IAB. I put the dwell meter on it and its was at a flat 6. It didn't move up or down at all. I tried turning it both ways and the dwell didn't change at all. This would mean its not in closed loop right? I figured it should be in closed loop since it had been running a while and the temp was up, the fan was even kicking on and off. All of the sudden the dwell started moving around like it was in closed loop. It did this for about 10 seconds and then it'd stop. It would keep repeating this series of event. When it did vary readings, it was about 15 which I believe is low. The dwell didn't seem to change much when I was turning the IAB screw, although it was hard to tell because it would just kick out of closed loop all of the sudden. Shouldn't it just stay in closed loop when the motor is this hot and has been running for so long. Any input on this would be a great help!

sugarsn10

iTrader: (2)

Any suggestions on why it won't stay in closed loop? As I said before I put in a new o2 sensor and coolant temperature sensor. I thought those were the primary sensors that allow the computer to enter closed loop so I don't see why it is acting so funny. I can't even get a good reading for the MCS with it barely staying in closed loop.

five7kid

iTrader: (14)

Do you have headers, or stock exhaust manifolds?

sugarsn10

iTrader: (2)

I just have stock exhaust manifolds. I may go to headers eventually. I've heard that with headers the O2 sensors don't get as hot so you should use a heated sensor, is that right? I guess that isn't my problem right now thought. I just can't figure why it keeps skipping in and out of closed loop.

The o2 sensor readings are really strange too. They are consistently about 0.045 volts which would be lean. If I close the choke flap I can get the readings to be around 0.3-.8 which is better. I currently have the mixture screws at 5 turns out and the IAB at about 2.5 turns out. This set-up would usually be considered rich yet the O2 readings are very lean. As I said before the O2 sensor is a new bosch sensor. The MCS clicks when I jump the two aldl terminals so I figured that it was fine. Any input is appreciated.

five7kid

iTrader: (14)

Shorty headers don't significantly affect the O2 sensor. Long tubes a different story.

Haven't heard glowing things said about Bosch O2 sensors in GM applications.

sugarsn10

iTrader: (2)

Do you think it be a good idea to try switching to a different brand of o2? They came with AC Delco from the factory correct? Do you think the Bosch sensor could be giving a weird reading to the computer which which is why its running so lean?

five7kid

iTrader: (14)

AC Delco, correct.

Can't say that's what's causing the problem for sure. Up to you whether you want to spend $50 to find out.

I'm pretty sure I put a BWD or some other non-AC/non-Bosch in my LG4 with shorty Hooker headers, never had a problem with it.

sugarsn10

iTrader: (2)

Alright thanks, I may end up trying that. If that's not the problem then I guess I have a spare when one of them goes out. Can the MCS be bad even if it cycles when you turn the key on? This is a re-manufactured carb, but I see that the MCS says GM and then the part number which would mean it's a factory piece so I have a feeling that it may not have been replaced when it was rebuilt.

sugarsn10

iTrader: (2)

I've read around and a lot of people say to check to see if the MCS works, you connect and B and A terminals and you should hear a fast clicking. Well I've done that multiples times and have always heard the clicking. Well today I had the EGR solenoid unplugged and jumped the two terminals and didn't hear the clicking anymore. Plug it back in and find the clicking coming from the EGR solenoid. So what's up with this? Any other ways to check the MCS besides the dwell, because as I said before it doesn't want to stay in closed loop. Any help is appreciated.

five7kid

iTrader: (14)

I suppose it's possible your EGR and MCS connectors got swapped. It would be hard to do, I think (I can't go look at them anymore, sorry).

The MCS should click for several seconds when you turn the ignition to "on" without starting the engine. You don't have to jump anything to find that out.

sugarsn10

iTrader: (2)

Alright thanks, well it only clicks once when I turn the key on, so do you think its the MCS itslef that is bad or a computer or wiring problem?

sugarsn10

iTrader: (2)

I just wanted to close this thread by explaining how the problem was fixed, that way if other people have similar symptoms they have a good idea of what the problem is in a lot less time than it took me.

I tested the MCS and knew it was good. All along I thought I heard it cycling, but what it turned out to be was the EGR solenoid. Big difference. I decided to take a chance and pull a computer from the junkyard in hopes that, that computer was good and my current computer was junk. Well, I put in the new $15 computer from the junkyard and everything works perfectly. The MCS cycles like it should and it runs like a dream set at the factory carburetor settings without tuning at all. So in short, my ECM was messed up and wasn't sending a signal to the MCS making it run like total crap. Thanks for all the help everybody!

five7kid

iTrader: (14)

Glad you found the problem.