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Chevy 350 TBI, running incredbily rich

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Chevy 350 TBI, running incredbily rich

LrsUlrich LrsUlrich
New User | Posts: 4 | Joined: 05/09
Posted: 05/18/09
07:48 AM

I have an '88 Blazer and when I got it the motor was shot. So I bought a rebuilt to replace it from a guy a town over. He warned me that it ran rich, and he wasn't kidding. I am currently getting about 8 MPG and when driving you can tell that the engine is flooding a little due to the way it stutters. I also have a problem where, especially on rainy days, I get quick moments where the engine will cut out then come back. It can happen up to about 5 times a minute. I've used products to get rid of the condensation in the tank but it still does it. I've had it to a garage a few times, but it never gave out any codes. Anybody have an ideas to what is going on?  

Guru | Posts: 1001 | Joined: 09/08
Posted: 05/18/09
09:25 AM

The cutting out in the rain seems like an ignition problem check for cracks in the coil etc.  Have you gone through the ignition?  What kind of shape are the injectors in?  

LrsUlrich LrsUlrich
New User | Posts: 4 | Joined: 05/09
Posted: 05/18/09
05:27 PM

The injectors seem to be in good condition. I'll be honest and say that I'm newer to cars, my father helps me out a lot. He's more of a carb guy, but he is a little familiar with TBI. Everything seems to be in working order...The ignition and everything was replaced when it we rebuilt. I've had some quarks with it, but most of them seemed to have worked themselves out. No one seems to know why I'm going through so much fuel. So to answer your question, as far as I know everything is in working order.  

55Guy 55Guy
Guru | Posts: 895 | Joined: 08/07
Posted: 05/19/09
06:28 AM

Are you getting a check engine light?

My first guess to your problem would be a bad O2 sensor (located on drivers side exhaust pipe just behind the exhaust manifold). If the O2 goes bad, that'll make the motor run pig rich. First check to make sure it's plugged in (common for them to be left unplugged in a motor swap.) If the O2 is good, then the next problem item could be the throttle position sensor (TPS), either it's bad or out of adjustment.

From there, it could be a stuck injector or maybe the computer has gone bad.

An O2 sensor for that truck is about $20, that's where I'd start. Also make sure the wiring leading to the plug is ok and not cut or frayed anywhere.  

LrsUlrich LrsUlrich
New User | Posts: 4 | Joined: 05/09
Posted: 05/19/09
06:22 PM

Yes I have a check engine light, but it does not come on till about 190 degrees. And as far as I know, that '88s don't have an O2 sensor. I have the stock manifolds on and there is no sensor. The TPS could be giving me the problem, it's just going to be a lot of tinkering to fix this.

Now if I'm wrong about the O2 sensor, please tell me. Cause from everything I've seen, an '88 350 TBI does not have one. But if I'm supposed to, it's not beyond my capabilities to install one.  

waynep712 waynep712
Enthusiast | Posts: 264 | Joined: 01/08
Posted: 05/22/09
07:49 PM

all tbi motors have an oxygen sensor installed in the exhaust pipe...... that is the only way it can set the fuel mixture...

it could also have a blown out fuel pressure regulator... in the throttle body housing..    when they blow out...  the dump gasoline right into the intake ...  

the throttle body gasket kit is under 10 bucks at most stores....  the fuel pressure regulator part is under 40 bucks...

depending on where you buy them....  you really need both... plan on a few hours... to do it...  most of the throttle body gasket sets come with new injector orings.. gaskets, and new injector filters that slip on with some work on the bottom of the injectors...   warning.. they do not come with new fuel line Orings...  large and small...  so you will need to find them in a fuel rail seal kit ... or a dorman help package...

when you put the replacement motor in... did you hook up all the ground wires...?????    the ones into the harness that leads back to the firewall behind the right hand exhaust manifold...      did you verify that the Oxygen sensor wiring did not get smashed between the motor and the transmission????? i have had to loosen a few motors and extract the wiring..

the check engine light coming on when it gets warm is a real signal that something is wrong...

i take it that you have installed a 192F thermostat...    i have had a few TBI motors in the last few weeks.. that were missing or had broken thermostats...   if the coolant temp does not come up above 155 degrees... it will stick at fast idle.. and won't come down for anything...

one last thing      does it have air injection system????// if the diverter valve is leaking air into the upstream (pipes into the exhaust manifold)  it will affect the oxygen sensor readings.. and make the engine run really bad...

the diverter valve controls the air between the upstream air, the down stream air and venting it to the atmosphere...

the upstream air only gets turned on during warm up...   once the oxygen sensor becomes active... the ecm turns off the upstream air... except during heavy acceleration ...    this prevents unburned air from messing up the exhaust stream gasses..  

the down stream air feeds the converter..  

LrsUlrich LrsUlrich
New User | Posts: 4 | Joined: 05/09
Posted: 11/13/09
09:25 PM

I realize it's been a few months since I posted this. I haven't been able to look at my motor for various reasons but I do like what you're telling me. I got the truck off an unpaid repo and the exhaust manifolds on it had no O2 sensors in it. The person who it was repoed off of, I was told, wasn't very car savy so I suppose the odds of that getting lost somewhere are somewhat high. Hopefully in the next few weeks I'll have time to look at it more. Before the engine mileage varied greatly. Now it has at least smoothed out to between 8-10. I realize my engine is tuned slightly for performance, but in cases where I shift down to take a downgrade, I can smell the engine load up on fuel. So I know there is a problem with too much fuel being added. And the thermostat is the stock one where it reads normal op temp at about 200ish. The check engine light still comes on but I'm not so sure if it's reality to the temperature or fuel problems because it has become very random over time. If you have any other advice I greatly appreciate it. Thanks.  

waynep7122 waynep7122
Guru | Posts: 1130 | Joined: 08/09
Posted: 11/14/09
10:02 AM

i would check the vacuum hose to the map sensor... make sure that it is free flowing when you take it off and blow through it...  it has to be hooked up to Manifold vacuum..   this is vacuum at idle...

i would also think that i would check for play in the throttle shaft... with the engine off and the throttle part way open.. see if it wiggles around ...  when it does... the throttle position sensor on the other end ends up at a slightly different position every time...       the throttle position sensor has to return to the exact same voltage every time... this lets the ecm know that you have  you foot off the throttle completely...    

worn throttle body bores cause a lot of problems...   they can be rebushed by better carb rebuilders...     the hard part to get is the screws...   they are tiny and metric...  2.5MM .45 pitch cheese head screws...   not something the average hardware store stocks...   i use a letter Y drill and some cut sections of a .375 thin wall valve guide to rebush them...  but i have a tiny stash of screws... the factory screw threads are deformed where they stick through and if you don't grind the exposed end off... you will break them off in the throttle shaft when you back them out...

if you have access to a scan tool.... you can verify that the throttle position sensor returns to the same spot every time in seconds...

here is a link to a list of data that every fuel injected gm owner should have..

it is what is normal  when you look at it with a scan tool...