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Chevy/GM TBI expert advise needed

This was posted also in a continuation of the “Unrepeatable Miracle … again” thread, however I wanted to draw from the experience and knowledge of those specifically familiar with the Chevy/GM TBI systems.

Update. I accessed the van’s self-diagnostics, and received a code which simply states the fuel mixture is too rich. I knew that already because of the black smoke, so that diagnostic code was not particularly useful, except that I did not get any other codes, which makes me think the problem is NOT the MAP sensor or the engine coolant sensor because if there is a problem with those units, there should be an associated code which is flashed.

I had always suspected that the O2 sensor might be going because before I moth-balled the vehicle, very occasionally the engine would start to emit black smoke and run terribly, and then that would stop and it would be OK again. So a few days ago I replaced the O2 sensor with a new Bosch unit. The engine started right up by turning the key and ran pretty well, only emitting white smoke (water vapor) because it was cold outside. I was hoping that as it warmed up the van would run great, but instead what happened was that the engine idle very gradually got slower and slower, and then it started emitting black smoke and the “check engine” light came on, and again the diagnostic was showing the same code as before … too rich. Now my brand new O2 sensor is fouled as well.

Next I visually inspected all the vacuum hoses that I could see well ( the 3 vacuum connections on the front of the TBI are very hard to see and access for they are right under the dash. I did find two hoses that were kind of cracked around the nipple, so I cut the bad part off and reattached. Then I pulled all my spark plugs AGAIN and cleaned them with carb cleaner and a toothbrush and a file and replaced them into the van. When I started up again, the engine seemed to run even better for yet a longer duration, but then again, gradually the idle got slower and slower, and then it started to smoke and the check engine light came on with the same code as before.

So now, my main suspect is the fuel pressure regulator diaphragm, which I didn’t even look at when I previously replaced the 2 tbi injectors. I took the regulator unit apart, but am not quite sure what to make of what I have found. There was some dried and hardened residual fuel residue which I cleaned out with carb cleaner. The rubber diaphragm itself seemed intact and certainly better than a picture I saw online where the rubber was stretched and sort of ballooned. However what I did find, was that the circular metal disc which sort of rocks back and forth in the diaphragm unit had uneven wear where that disc contacts the end of a metal tube for the purpose of blocking the fuel flow. So I am wondering if fuel may be slipping by when that should be completely shut, and perhaps that is causing the problem.

I’ve already dropped almost $300 on parts and fuel tank cleaning, and am not eager to spend yet $30 more on a regulator diaphragm which may or may not solve the “running too rich” problem. Any thoughts from anyone as to what I should do from here ? The behavior of the engine is very consistent. If it hasn’t been started for a day or so, the engine starts right up and seems to run quite nicely for a while, but then the idle speed gradually gets slower and slower, and then it starts to emit black smoke and the diagnostic emits the “running rich” code.
Thanks for any help you can give.

A throttle plate cleaning and MAF cleaning would be a cheap place to start, along with a dose of tecron or seafome. Lets get some basic maintenance info for air filter gas filter and plugs.

Hi Barkydog. I don’t believe this vehecle has a MAF sensor. The vehicle has a new in tank fuel pump & fuel sock. It has new plugs although they are already somewhat fouled from the too rich mixture. New but somewhat fouled O2 sensor. The throttle plates and tbi have been sprayed many times with carb cleaner.

I have limited experience with the TBI system but I do know that the mixture is very closely controlled by the ECM. If the ECM is receiving wonky information from any engine component then the mixture could be very lean or very rich. The IAC is another component that I would look at as well as the TPS. I think that’s where your problem is…it’s just a matter of checking them out.

Thanks missileman. I’m going to read up on the IAC and TPS now.

Carb cleaner is the wrong thing to use as it strips a protective finish, thus throttle body cleaner. So I do not post irrelevant info, please post the year of the vehicle, miles since plugs, air filter and gas filter if applicable. Throttle body assmebly should be removed to clean, as best as I know preferred workflow. Leaking injectors may be another issue, seafome or techron could be worth a shot, don’t be too hard on us, without the car to look at it is a matter of most probable cause. Reference

ok, according to my Haynes manual, if it was the TPS the diagnostic system should definitely be sending a code indicating a TPS malfunction, and that is not happening. The IAC might be the problem, but the replacement part is even a bit more $ than a replacement fuel regulator diaphragm. … and I’d just be guessing with either one. Also if I understand the IAC’s function correctly, it only comes into play while the throttle plates are completely shut, so theoretically if the plate is open (my foot on the gas a bit) the car should run fine. However if anything, depressing the gas pedal only seems to make the engine smoke and run even worse.

Try replacing the fuel pressure regulator diaphram.

There’s only two ways excessive fuel can enter a fuel injected engine. Either thru the injectors, or thru a defective fuel pressure regulator.


That’s my hunch as well, but I have to remind myself that other systems’ malfunctions can make the ECM issue the wrong orders, so it may be through the injectors, though they themselves may be OK, … or not. btw the injectors I just replaced with re-manufactured units.

Those were rudimentary systems. The most common reason for running fine cold but rich hot was due to the temp sensor for the ECM (versus gauge). Seen them fail many times and not set a code. The ECM just considers it cold and maintains enrichment mode. Cheapest part swap you can make. What do you have to lose at this point?

Wow. I hope you become the champion of this thread !

Hi Twin Turbo. Well I replaced the coolant temperature sensor, and it wasn’t easy because a support bracket to the alternator was so close to the sensor that the socket wrench couldn’t fit between that bracket and the sensor. With great difficulty I managed to use a small crescent wrench to tighten up the new unit (not enough room for a regular box wrench of that size)… I used a socket to get the old one out, but broke the plastic electrical connector doing it (because of that bracket).

Unfortunately I can’t say right now whether you were right or wrong about the temp sensor being the cause of running rich while warm, because the engine is now behaving differently.
Whereas before the engine would keep running for as much as ten minutes before crapping out, now it probably won’t even go for one minute. The explanation must be that I took apart the regulator assembly to examine it, and put it back together. I am suspecting that the metal disk which rocks back and forth on one side of the regulator diaphragm, and which had shown a semi-circle of wear where it contacts the end of a fuel tube built into the injector cover-plate, is now not contacting that fuel tube where it used to, and therefore fuel is slipping by. I’m worried that if I simply replace the regulator diaphragm, there may be wear on the end of the fuel tube (which is built into the injector cover-plate), which may not allow a tight seal with the new disk on a new diaphragm. Can anyone tell me whether the semi-circle of wear that I see on the disk is a typical problem of a diaphragm gone bad? Thanks.

The MAP will sense hard acceleration and increase the fuel flow if there is a vacuum leak or if the exhaust is restricted, causing weak vacuum.

The idle air control can be cleaned with carburetor cleaner sprayed into the odd key hole at the 2;00 position on the throttle body.

Look into the throttle body with the engine idling. There should be a fine mist exiting each injector in an umbrella pattern. If fuel is dribbling out there is a leak.

My best guess is a faulty MAP signal from one of the causes mentioned above, though.

I agree with @TwinTurbo. A faulty CTS will cause these symptom, and it will not usually throw a code.

EDIT: I posted the above before seeing you already replaced the CTS (hopefully, you replaced the right one). The fuel pressure regular is the next likely suspect.

Thanks everyone. I’ve ordered a new regulator diaphragm. It will probably be a week or so until I get It. Will post back then.

Hope its not like the boss’s Chevy,they messed around and himmed and hawed with it replaced the TBI and still no dice so they replaced the ignition componets,son of gun,ran like a new one,I know the Throttle body is supposed to spray a conical pattern and if its just dribbling there are issues-Kevin

Yes, the injector spray does not look good to me. There is dribbling as well as a conical pattern. If the ECM was sending bad orders (too rich), even then I would think the appearance of the spray should be good for the ECM simply orders a ball valve to be open or closed. Too much fuel pressure however would affect the quality of the spray pattern. … I think

Have you measured the fuel pressure? I know it’s a PITA on these, I had to make up my own out of parts. They sell them but it wasn’t off the shelf at the parts store and I needed it quickly.

Also, on some models, there is a sintered filter in the TB at the fuel inlet. I had one with and one without. Might want to verify if there is one, take it out and see if it’s clogged up.

Bad spray pattern = first step, check fuel pressure and volume.

@yuyakay, did you remove the TBI assembly from the engine or repair it under the hood? What parts were replaced in the TBI?

@Rod Knox, just the injectors and injector o-rings, … so far. the filters around the injectors are also new.