Shops don't scan OBD1 cars for codes anymore. Only OBD2. Your car is an OBD1 car, that's why they don't know what code it is, but there is definitely a code there, that is indicated by the stumble, fans on, SES light.

As mentioned earlier, you need to download the correct scanning software to a laptop, order the correct cable and read your own codes. Its really simple and a must have with these cars.

 

Only a terrible shop won't scan an OBDI car.

 

Thanks for the info and I transferred your info to Pep Boys where they claim they can properly diagnose with their equipment. I am most skeptical however. They said code 36 cam up as the only code. They are still stumped.

Again...heres the situation. I start the car, drive for a few minutes and then there is a stumble or hesitation and the SES light comes on and the aux fans start blaring. Once the SES light comes on the car runs fine. If I start the car cold and the SES light is not on and driven a low speeds it will stall while driving. Seems it doesnt have the fuel pressure to continue at such low rpms. I have to pull over and again start the car. Car does not start quickly....many times the gas pedal has to be vigorously pumped to get it going. Once it gets going it again runs fine.

I have kept trips to a minimum because of this and the threat of accident. A car dieing on a highway is not too healthy. One thing I did recently notice is if I made many stops running errands and the car was warmed up sometimes no SES light would come on when started and ran fine without the SES light ever coming on. Go figure.

I recently had my car in for inspection and when on the lift I noticed the charcoal canister hanging in position by only the tubes. The mounting bracket had rusted away leaving it hanging. The shop did me a favor by reinstalling the canister to its recommended position but now Im wondering if this is when the problems started. If the tubes or canister was damage in any way could it cause these symptoms? Tube blockage with charcoal for instance??

Thanks again for all your help. I am far from being a mechanic but am somewhat intelligent and learn quickly. You forums and information is very educational

 

Pumping the gas pedal doesn't really do anything other than let some extra air in. It's not like a carb where there is a pump that pushes extra fuel into the manifold when the pedal is pumped.

The problem might be leaking injectors (or another cause) causing it to flood. Before turning the key to "start", put the accel pedal on the floor and hold it there until the engine starts. That puts the PCM in "clear flood" mode. If it starts easier, it's flooding for some reason.

As mentioned, could be leaking injectors. Could also be a problem with the coolant temp sensor. That's what the PCM looks at to set the startup A/F ratio, richening the mixture like a choke would when the engine is cold.

I'm wondering if you have an intermittent problem with the coolant temp sensor. If the circuit is open, the resistance is infinite, and the PCM interprets that as a temperature below -40*F (40* below 0). It would be too rich and start hard if it wasn't too cold. As the engine warmed up, it would get worse, but the PCM would set code DTC 15. the fans would come on and the engine would run better.

If you want to check the coolant temp sensor, Shoebox has the instructions.

 

Thanks again. Everything you say points to the coolant sensor and I am going to replace that. Where is the best place to get the sensors? Should I go OEM or are there better aftermarket parts??

 

Here is Pep Boys response to me:

We utilize the latest technology available today which includes the Snap-On Verus. Please inform the Z28 Forums members that you have access to a Snap-On computer and the code is a 36. This Snap-On Verus is a $10K laptop with all the cables for all OBDI and OBDII Cars. Please note that All-Data provided a trouble tree which stated clear the code and if it comes back on to continue the problem is that the code does not come back right away.

This can be a fuel issue. When vehicle is KOER the fuel pressure needs to be 41 to 47 and not 37/38. Please let the forum know that and when KOEO the vehcile looses pressure.

Please let the forum community know and get back to me.


Can you please comment that we are on the right track??

 

With regard to the fuel pressure they are not completely correct. The pressure with the engine idling at closed throttle should be 43.5psi. GM factory manual indicates that pressure in the range of 41 - 47psi is within spec. What has not neen stated is that is the pressure with the vacuum compensation line removed from the fuel pressure regulator. When you reattach the vacuum line the rail pressure will drop proportional to intake manifold vacuum. A drop of 8psi from the "no vacuum" reading is typical with a stock cam. You will see less pressure drop with a more aggressive camshaft.

The pressure should also be tested with the engine under max load - wide open throttle above 5,000 RPM. It should not drop below 40psi.

Yes, the pressure will drop when the fuel pump shuts down. I believe the factory manual requires that the pressure not drop more than 5psi in 10 minutes.

 

Why replace parts without testing? That's not the way to do it.

 

Thanks for your reply. I have passed your comments on to pep Boys.

The coolant sensor just seems like it is the problem. In matter of fact years ago this was disconnected at a shop without my knowledge. I only noticed it recently when I went to change my battery and saw the disconection. I have reason to believe that this was done to cover their butts on other electrical work , opti etc, which may have not been needed. No proof...just a hunch. I reconnected it and noticed the wiring harness connector had grime and what looked like acid build up.....like the diodes of a battery. I tried cleaning the best I could.

When I had the recent SES problem I disconnected it again thinking that the sensor was sending a false signal to the computer thus enabling the aux fans. Is this why the shop disconnected it? I dont know. I told this to Pep Boys and they never reconnected it. In reading your post about what the sensor does and how it can affect the gas mixture ratio etc this all made sense.

What do you think??

 

We are talking about the temp sensor in the water pump, right?

If it's disconnected it should set a code. I bet GaryDoug's FREE (not $10,000) scan software download would pick it up. .

Key on, harness connector off, make sure you have 5V between the two wires. Still no harness, measure the resistance between the two pins of the sensor. The resistance should reflect the temperature per a table on Shoebox's page. I'd post a link but I'm on a phone and I have fat fingers.

 

ha ha I'll tell them that.

The sensor that was disconnected is I think called the coolant level indicator module, I didnt know one was in the WP. I'll bring that up too. If the coolant module is disconnected, could that activate the SES light??

Thanks again !!!!

 

The coolant level switch is on the passenger side of the radiator a few inches below the fill neck. It has no effect on how the engine runs, is not connected to the PCM and would not set a code or turn on the fans. Very common for the switch to fail. Some people don't want to replace it so they unplug it to turn off the dash light.

What is the "coolant moduke"? I've never heard of that.

 

If you want to check the coolant temp sensor, Shoebox has the instructions.
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Could you possibly post this link? I'm having a hard time finding it...thanks

 

Im interested in having the coolant sensor replaced in my 94z. I have very little mechanical experience but would like to try and replace it myself. I know its located either on or by the water pump. Does the water ump have to be removed to replace this sensor?

Thanks for your help

 

Here's how to test it:

4th Gen LT1 F-body Tech Articles

Here's the location (93-97 are the same):



No need to do anything to the water pump. You unplug the harness, and unscrew the sensor with a wrench. BUT...... coolant will escape when you remove the sensor. DO NOT LET THE COOLANT DRIP ON THE OPTISPARK DISTRIBUTOR. Pack rags around it.

You could drain the radiator - petcock at the bottom, passenger side - so the level is below the coolant sensor, or have the new sensor all ready to screw in and do a quick swap (easier said than done).



After you get the sensor back in, refill the lost coolant and bleed the air out of the system.

4th Gen LT1 F-body Tech Articles

Bookmark Rob's site..... and use it

4th Gen LT1 F-Body Tech Aids