How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site. Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Joe Your Own Question
Joe, ASE Certified Technician
Category: Chevy
Satisfied Customers: 7251
Experience:  ASE, electronics, Marine inboards and electronics, automotive drivability, all around repair tech,
Type Your Chevy Question Here...
Joe is online now
A new question is answered every 9 seconds

94 chevy: 1500 did a tune up and map sensor

Resolved Question:

got a 94 chevy pu 1500 4x4 it has the 350 is at the shop right now, and they can't figure out what to do, it ran real rich, so they did a tune up and map sensor. It starts up great and then starts running real rich, too much fuel, where do we go from here?
Submitted: 7 years ago.
Category: Chevy
Expert:  AMEDEE replied 7 years ago.



I would recommend to check fuel pressure, check the exhaust for back pressure and engine vacuum at idle as well as ignition timing. These things should get you pointed in the right direction.

Customer: replied 7 years ago.
Not even sure if these guys no how to check exhaust back pressure, i don't. If the fuel pressure is too much or less what does that indicate? I don't think it is the other stuff, it starts out fine, then it loads up
Expert:  AMEDEE replied 7 years ago.

If the fuel pressure is too high, this might indicate a pinched fuel return line.


If the exhaust was plugged up, just pull the o2 sensor and see if that helps.

Expert:  Joe replied 7 years ago.



A very common problem with these is the fuel injectors leaking and a weak fuel pressure regulator spring, the throttle body rebuild kit costs $55 at napa auto parts and make sure you do not loose the tiny filters for the injectors, the fp regulator spring is another $70, remove the air cleaner and watch the injectors while the engine is running and they should spray a fine cone mist. Check the fuel pressure with a fuel pressure tester and a tbi fuel system adaptor and should read 9-13psi however 14 psi is also common.




Customer: replied 7 years ago.
I just saw your last reply, i think we are the same page now!! thanks
Customer: replied 7 years ago.
oh, i see you are a different mech. I like your reply, did you see the other replys? just wondering how that works? thanks tim
Expert:  Joe replied 7 years ago.

Yes, as reason I tried to give you some other things to check. The most common problem on those tbi engines has to do with the throttle body. But very good to also check your timing anyways as well. To check your timing on that truck remove the black cover on the passenger side firewall and then disconnect the single wire connector and then check your timing after you check and or adjust it once done turn off the engine and connect the wire back together.


If you need any further assistance I will be glad to help you.




Expert:  AMEDEE replied 7 years ago.

Hello there!


Sorry for the delay!


I did some more digging for you and have some things for you to test to determine the cause of the engine running too rich.


Obviously, be sure to check the engine coolant temp sensor before getting into all sorts of electrical diagnoses. The engine running rich could be something as simple as that!


If that checks out, you could have a bad or lazy oxygen sensor. The heated oxygen sensor is very important to control not only the cars emissions but it also helps the vehicles performance! If it is not working properly it can result in a number of problems like rough running stalling hesitating bucking and jerking. But more common than that if it is lazy, the cars gas mileage can drop considerably, and sometimes not even set a check engine light on the dash!
The fuel control Heated Oxygen Sensor (HO2S 1) is mounted in the exhaust manifold where it can monitor the oxygen content of the exhaust gas stream. The oxygen present in the exhaust gas reacts with the sensor to produce a voltage output. This voltage should constantly fluctuate from approximately 100 mV (high oxygen content - lean mixture) to 900 mV (low oxygen content - rich mixture). The heated oxygen sensor voltage can be monitored with a scan tool. By monitoring the voltage output of the oxygen sensor, the PCM calculates what fuel mixture command to give to the injectors.




1. Verify with a spark tester that the tester can be fired with it adjusted to 25 kv.

2. Try adding fuel, does the additional fuel help this accelerate.

3. With the timing light, verify the base timing and then what happens with the timing on acceleration when the problem happens.

4. Verify fuel pressure and monitor the spray pattern of both injectors.


Potential Causes:


Engine Mechanical
Exhaust Gas Recirculation (EGR) Valve
Fuel Injectors
Fuel Quality
Secondary Ignition System





1. Disconnect the injectors and run engine on carb spray or propane to see if it will run good.

2. If not check the Exhaust Gas Recirculation (EGR) valve for seating, secondary ignition and engine mechanical condition with a compression test.

3. If runs good on alternate fuel, check fuel quality and flow bench test the injectors.



Expert:  AMEDEE replied 7 years ago.

Also, here is a really good test that will point you in the right direction.


Here it is. Its called the Flat raters test drive.


The flat raters test drive..........


Under safe road conditions, a wide open throttle acceleration is performed. Try to maintain WOT for at least 5 seconds. 10 to 20 seconds would be better.


What will the pre cats do?

Will indicate rich

Should the barometric pid change?

Should remain stable! Very important!

What will air flow do?

Max out. Volumetric efficiency. Engine load 100%
(Engine load Parameter on scan tool should go close to 100%)



Poor acceleration, rich exhaust, and good Barometric pressure = restricted exhaust or late ignition timing.


Poor acceleration, rich exhaust, low or decreasing Barometric pressure = intake path restricted


Poor acceleration lean exhaust and good barometric pressure = fuel delivery concern.


Poor acceleration, lean exhaust and low barometric pressure = air flow measurement error. (Ford has this problem with Mass air flow sensors)


Note: Some vehicles Barometric pressure is not updated until Wide open throttle.

Expert:  Joe replied 7 years ago.

Well I guess after I gave you some actuall things to test and how to inspect them the other expert has decided to actually try to help you, however I see no reason for you to run this engine on carb spray or propane, that is very impractical for a customer to try and do and sorry for that info. Yes do check your coolant temp sensor as well however if the engine is running rich once warmed up it would not really be that sensor.


If you still have problems and would like assistance with your question I will be glad to help you as I own 2 of these trucks and know the tbi system inside and out so I will be glad to assist you in finding the problem.


Thanks again,


Expert:  AMEDEE replied 7 years ago.

I apologize sincerely for "all around tech". As you can see, he is a newbie around here and has a lot to learn. I am sure once the moderators get a hold of this thread they will not be to happy with him. I was the first expert you got a response from..... "all around tech" is just trying to stick his nose where is does not belong.

Expert:  Joe replied 7 years ago.

Is the check engine light on and if so do you know the codes?


I am very sorry for this seeming unprofessional, this is not how business is conducted on here and very sorry you had to see that.


If you do actually want further help I would be glad to help you.

Expert:  AMEDEE replied 7 years ago.
And I will be more than happy to finish what WE initially started...Let me know!
Expert:  Joe replied 7 years ago.

Hello Boys 3,


Just so you know I am opting out and have contacted support about this problem as it is very dangerous to run off propane as your plugs are fouled from gas and will not fire right and even without that if you run it off propane or a carb spray and it where to backfire it could seriously hurt or kill a person if it backfires and there is an explosion, So please whatever you do, do NOT try to run this engine on propane or carb spray. I do not want to see you possibly get hurt.


I am going to ask for you to please ignore the problems here.


Thanks and sorry for the unprofessionalizing.



Joe, ASE Certified Technician
Category: Chevy
Satisfied Customers: 7251
Experience: ASE, electronics, Marine inboards and electronics, automotive drivability, all around repair tech,
Joe and 5 other Chevy Specialists are ready to help you
Expert:  AMEDEE replied 7 years ago.

Since "All around tech" finally took his place, we can continue our conversation in peace.


This test that I have given you is the correct test to do in this particular situation. This test was designed from seasoned techs in this business for an engine thats running rich. Only rookies don't know about this.


But yes... obviously be careful when doing any test under the hood.


Please let me know what you find and we can pick up where we left off.

Customer: replied 7 years ago.
I do appreciate your help, but way too technical for mech looked at the throttle body and that was the problem, it was dumping fuel into it...i have used you before with great success..i would like to pass on this one ,forgive me...tim
Expert:  AMEDEE replied 7 years ago.



Gald to hear its fixed!



Customer: replied 7 years ago.
Hi, back again...i still have a skipping problem and although it runs better it still is far from right...we noticed the dumping of fuel , after changing temp sensor it got the fuel adjusted but it still seems to skip and will stall when at idle, almost like now it is getting too little fuel. boy I wish you could see it...confused!! what troubles me most is the way it skips or runs rough even when giving it fuel.The truck sat all summer but it ran great before,you mentioned timing, but there is no way it can get off is there?help
Expert:  AMEDEE replied 7 years ago.

When was the last time the engine was tuned up? No, not just plugs, but a full tune up! The whole nine yards! Any part of the secondary ignition will do this including the ignition coil or coils! This sounds like a classic ignition misfire! They usually go bad under a load like under acceleration, but can be noticeable at any time. It is very important to have your engine equipped with good tune up parts or you may be causing damage to other components of you car like the catalytic converter which can be expensive!

Possible causes:

Ignition misfire causes by any bad tune up part and or ignition coil or coils.
Bad spark plugs, spark plug wires and cap and rotor (if equipped)
Wrong tune up parts spark plug, wires etc...

Insulator cracks.
Improper gap.
Burned electrodes.
Heavy deposits.

The quickest way to check the ignition components, is by looking closely for leaking secondary voltage. Visual look at all the tune up parts to see if you can see any sparks coming from any of the wires or coil (or coils) when the vehicle is running. If not, you can put your hand on the coil/coils and wires when the engine is running and see if you feel any small voltage leaking form the tune up parts. Check spark plug wires by connecting ohmmeter to ends of each wire in question. If meter reads over 30,000 ohms, replace wire(s).
ALso, with engine running, spray coils and plug wires with fine water mist to check for shorts.

It will leak secondary ignition voltage from the tune up parts caused by high resistance from carbon tracking if they are bad. This is very common if the tune up parts are not changed regularly!

If any is found, the tune up parts are simple bad an will need to be replaced! If none is found than you can spray a mist of water on the tune up parts when the engine is running and see if that causes the engine to stumble or run rough at all. Again, if it does, the tune up parts are bad and will need to be replaced!



Customer: replied 7 years ago.
I didn't mention that the local garage did the whole tuneup and gave up after that didn't work, i have it with another guy now trying to help but he has little for diagnostic tools. So i guess look at the coils?
Expert:  AMEDEE replied 7 years ago.

THat sounds like a good idea.


Also, lets not forget about the pickup sensor inside the distributor.


IN addition, lets be sure engine vacuum is up to par and that there is no vacuum leaks.

Expert:  Joe replied 7 years ago.



So what has been done up till this point?


If you put the truck in gear while running and hold your foot on the brake and rev up to above 1200 rpms or so can you hear and feel a missfire under the load?


Have you tested for which cylinders are missfiring yet, this can be done by attaching a timing light to each plug wire one by one and let me know which cylinder if any are actually missfiring because before we start guessing at all the things that could be wrong I would like to be 100% positive of which system has a problem if it is a true ignition missfire then it will show up on a timing light.


Let me know what you find you have already paid for the service from me so even though I opted out because of the other experts unprofessionalizem I will be more then happy to further assist you with complete testing, I will not just tell you to check this or check that I will be more then happy to actually give you things to check and the correct values.


Thanks for your understanding,


Customer: replied 7 years ago.
Well we got it.... it was the temp sensor, the skipping was due to the excessive gas it dumped into the cyl. The new plugs couldn't handle it ...another set of plugs and it runs great.....thanks for the help
Expert:  Joe replied 7 years ago.

No problem glad to hear that the rebuild kit for the throttle body and a new tps fixed it for you.....thanks for using justanswer!



Customer: replied 7 years ago.
did you get compensated? not sure who got the payment...thanks tim
Expert:  Joe replied 7 years ago.

Yes Tim I was,


thank you