New member here looking for some help with 350 TBI running rich

[gfritz25]

Hi guys. New member here, and I'm looking for some help. I have an 87 GMC 2500 with a TBI system on a 350. The engine is running rich, which I have tried to correct. I have replaced the MAP sensor, and EGR valve, which were both throwing codes. I have also replaced my IAC valve, TPS, and oxygen sensor. Tune-Up was done recently, spark plugs, wires, dist cap, fuel filter, but I have not done the coil yet. It's still running rich, which is a problem. I have read that it could be anything from the fuel pressure regulator, timing being slightly off, coolant temp sensor, ect... Trying to get this taken care of as this is my daily driver. It isn't running rich enough to be blowing any black smoke or anything like that, but there is a very noticeable fuel smell from the exhaust. Any ideas on what it could be, or anything I may have overlooked? Thanks in advance.

[gfritz25]

Also. The truck is idling really low when I put it into gear from a cold start. Kinda sounds like it has a big cam in it, which it doesn't.

[Irish_Alley]

Guessing I'm going to say the cts. There's testing on the tbi sensors that I made a post about.
Here in the technical section

[pholliday1]

Unfortunately you have dreaded tbi surge. If you read other posts similar to this it's kinda the same thing, usually the poster throws every part imaginable at the motor but usually the problem exists. However there are a couple of things to check. One is to check methodically for vacuum leaks around ENTIRE manifold and tbi using the carb cleaner method. Also these trucks are notorious for having distributor problems that don't "APPEAR" to have issues. Another thing is to get your hands on a thermal temp gun and with the motor cold, check each manifold port for LARGE temp differences. I have 5 tbi trucks ,recently I had a bad distributor that "looked" fine and I had a broken plug wire inside the boot also even though the wires were relatively new. Sometimes the distributor can do funky things and that's why it's so hard to troubleshoot.

[roundhouse]

The water temp sender going bad will usually make it run rich


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[bd]

Also. The truck is idling really low when I put it into gear from a cold start. Kinda sounds like it has a big cam in it, which it doesn't.

Confirm that timing is adjusted to TDC with ESC bypassed.  Does the "SES" light work?  Have any trouble codes recorded since replacing MAP & EGR? 

Check the fuel pressure.  With steady throttle, fuel rail pressure should remain stable at ~11-12 PSI, momentarily jumping to ~15 PSI with sudden full throttle opening.  Check engine vacuum with a gauge and post the results.  Is the vacuum gauge indication steady or bouncy?  Check for exhaust leaks upstream from the O₂ sensor.

[frotosride]

When I get a chance I will adjust my TBI to surge again and post a clip. If it's what you are experiencing then let me know and I will tell you how to correct it

[gfritz25]

Have not yet confirmed that timing is set to TDC. Don't have a timing light to confirm this. Going to see what I can do about checking the fuel pressure, I don't have a fuel pressure gauge, but my buddy does. No codes since MAP and EGR have been replaced. Possible exhaust leak upstream from the O2 sensor. I haven't noticed any vaccuum leaks, but I have been watching for them. I don't think my dist is bad. Making me think it's the CTS. Gonna see about replacing it this weekend, and get back with you guys. Thanks

[bd]

...Possible exhaust leak upstream from the O2 sensor.  I haven't noticed any vacuum leaks, but I have been watching for them....

An exhaust leak can be a double edged sword.  Not only will exhaust gas blow out through any opening, but air can be drawn into the exhaust stream via aspiration, increasing the oxygen content of the exhaust.  An O₂ sensor cannot discern between oxygen enrichment from an outside source and unspent combustion gas - both will result in fuel enrichment.

Finding a vacuum leak involves spraying around intake gaskets, the throttle body base gasket, both ends of the throttle shaft, and vacuum hoses, using aerosol carburetor cleaner.  Exercise extreme care to avoid spraying directly at the distributor or puddling of the spray on and around the manifold, since it is highly flammable.  Wipe up puddles of fluid with a rag, immediately and have a charged fire extinguisher at hand and ready for use.

As long as base ignition timing is between TDC and 6° BTDC, there shouldn't be any adverse consequences.

By all means borrow a fuel pressure gauge and vacuum gauge as earlier suggested.  The vacuum gauge should help determine whether a cam lobe is wearing flat.

[gfritz25]

Well I didn't get a chance to get to the CTS this week, but I did get a chance to block off my fuel return line into the tank selector valve. Started idling smoother. Thinking possibly fuel pressure regulator... What do you guys think?

[bd]

Checking the fuel pressure with a suitable gauge will provide the answer.  Could be the pressure regulator, fuel pump, or a restriction such as a plugged in-line fuel filter, plugged tank filter sock, severely kinked hose or sticking tank selector valve.  Measure the fuel pressure before throwing more parts at it.  It's not difficult to do with the right adapters.  If you keep the truck it will benefit you to own a pressure gauge.

[gfritz25]

Also. Forgot to mention that I had my scanner on it this weekend. Scanner pulled that the ECU said it's running rich, which would rule out the o2 sensor, and exhaust leak, correct? If there was a leak, it would be pulling air in the leak, telling the ECU from the o2 sensor it's running lean, and to dump more fuel in, right? Knows it's running rich, so I'm thinking it has to be something mechanical...

[gfritz25]

Just an update for you gentleman. Fuel pressure is okay. A little low, around 10-11 PSI, but shouldn't be enough to affect anything huge. Checked and replaced the CTS, which helped a little bit, but didn't fix it. Also got the timing set back to 0° BTCD, which helped again, but still running rich. Found a vaccuum leak at my throttle body spacer, which I got a gasket to take care of, and gonna do that probably tomorrow. I'll keep you guys updated as I can, although I start school on the 31st, so it may be a little difficult to keep up with all this. Thanks for the help so far guys, seriously appreciate it.