Any ideas what to look at to take care of a cold hesitation problem? It stumbles badly on the way up to speed but smooths out when it reaches operating temp. Car has 82K on it.

 

Did u ever figure out your problem ?

 

I'm having the EXACT same problem with my 85 and no one on here has replied with any help other than to buy the FSM. Did that and it wasn't much help. Scanned through tons of these threads and by reading them the OPPOSITE seems to happen when the Oxygen sensor goes bad. I'm at a loss as to what to do myself, so unless anyone has had this problem before and responds to us, I'm going to replace the Oxygen sensor and the coolant temperature sensor (the one on the water pump) and keep my fingers crossed.

Scott

 

Have you or the PO bypassed the throttle body?

 

The oxygen sensor is not used until the engine goes into closed loop. Check your computer temp sensor (Frt of eng, the smaller one, the larger one is for the cold start injector, and is obviously working.), Fuel filter, and fuel pressure are other common problems.

 

OK, thanx for the info on the Oxygen sensor...Will definately be checking the computer temp sensorl. Also, I've had to replace the fuel pump on this car twice in the last 8 years so I was wondering about the fuel issue. Any idea on how this could be affecting the car when it is cold?

 

OK, I'll throw out a theory. Your engine is stumbling only during cold, so your engine presumably in "open loop" operation. You may have a faulty MAF or MAP sensor, which is causing the PCM to calculate incorrect pulse width's for the injectors(ie your are getting either to lean or rich of an AFR). During open loop, the PCM has no idea if it has achieved the correct AFR, and this is due to no feedback system, which is the job of your O2 sensors. The AFR is being calculated solely on the positive feedback of the MAF/MAP.

Now, when your engine smooths out when it is hot, the O2 sensors are now brought into the AFR equation, or "closed loop" operation. Now that the O2 sensors can feedback the oxygen content of your exhaust, the PCM can now make short term fuel adjustment to correct the AF ratio. So I am theorizng that the faulty,or out of spec. sensor is being succesfully compensated for by your O2 sensors....

gccsv ==>"the OPPOSITE seems to happen when the Oxygen sensor goes bad".
Correct, a bad O2 sensor will foul up the AFR calculations during "closed loop" operation, and have no effect during cold operation. However, keep in mind that the O2 sensor feedback to the PCM is ignored during WOT even during "closed loop".


Just a theory.....(the MAP/MAF are used on LT1's, and I am not sure what your L98 uses for AFR management, but just apply to your systems sensors.)

 

MikeC4 was correct about my problem: MAF. After fighting with a bad check engine light circuit I got the diagnostics working & got a 33. Thanx man.

Scott

 

Glad to hear it! Let us know how your engine runs "cold" after you install your new MAF sensor....

 

It just keeps getting better...Put MAF on, no more code 33. After driving the car for about 20 min the SES light came on again. Went home & got a code 45. Didn't need the computer to tell me it was running WAAAY too rich...Fumes, plus I was getting 11 MPG according to the gauge and even if you pressed on the accelerator it would still stay around 11 mpg cruising at 55. Ohmed injectors last night while warm...All ranged from 19.3 to 21.7 . Will be doing them again today cold. It "seems" like I don't have a lack of fuel, but with the way these computers work, maybe I do. I'm starting to wonder about my ECM. It was too hot to pull the o2 sensor per the fuel problem chart in the FSM, but will do it here shortly. Complete exhaust replaced 4 years ago so I don't think I have any exhaust restrictions. HELP!

 

90, I'm thinking yours might be the coolant temp sensor.

gccsv, sounds very typical of a bad 02 sensor now.

 

Check the plugs and tell us what they look like.

 

just pulled them...Fuel fouled, all of them.

 

11mpg, you are runing way rich indeed. Now be careful not to "shoot the messenger" and immediately condem your O2 sensor. Your engine is running rich, and your O2 sensor error code 45 is reporting back to the ECM that you are running rich(700mv or higher more than 30 seconds in "closed loop" operation). Also, your plugs are obviously idicating rich condition.

So, in a perfect world,your ECM should trim your fuel using the O2 feedback and "lean out" your AFR. But it seems as though you are running rich at all times, so something is prohibiting your system from running and maintaining the magical "stoichioemetric" AFR of 14.7/1 ....(rich would mean something like an Air/Fuel of 14.4/1 which translates to LESS air per unit fuel. A lean condition would be something like 14.9/1 or MORE Air to unit fuel)

Assuming your MAF/MAP are working properly:
-check that you do not have excessive fuel pressure in your fuel rail. This would cause an uncontrollable rich condition by forcing more fuel than intended when your FI's open. Think of your FI like a sprayy nozzle at the end of a garden hose. If you put the water pressure low, and squeeze handle for 1 second and release, you will not get that big of spray of water. Turn up the water pressure, squeeze the handle for 1 second and release, and you get much more water during that short time frame. Since ALL of your plugs are fuel fouled, I would look at this first.

-By all means, remove and inspect your O2 sensor. Just don't be convinced that this is the culprit.

 

OK. Had to order a fuel pressure gauge so when I finally get it I'll be thoroughly checking the fuel system. Checked most everything I could short of checking pressure of everything. I'm gonna check the O2 sensor too. Did ohm the injectors & got in the 19.3-21.7 range. That seems high. All the posts I read here show 17 or lower & way lower when there's a problem. Unless the prob is with the O2 sensor, I won't be messing with the car until next week...Well, maybe a little...

*** Didn't mean to steal this thread but it just kinda happened... ***

 

90rdstr must not have minded the hijack ....he never replied...

you own it now!!!!

 

Mike, do you know what the higher than what I've seen numbers from the injectors could mean? I'm pretty sure these are the original ones...and about 89,000 on the car.

 

Sorry, I am not sure what these numbers mean, but they may be pulse width times, which would mean more fuel through the injectors??

Question #1, does your engine run rich during cold start as well as when it is hot? If so, this may indicate your O2 sensor is OK, because it is NOT used for AFR calculations until engine hot.

Question #2, can you easily remove your fuel rail to see if you have leaky injectors? On my LT1, I could remove fuel rail assembly in about 10minutes. An easier way would be to hook up your FP gauge, turn ignition to "on", let the pressure build to peak, ignition "off", watch to see if pressure drops noticably over a short period of time(let's say 10 or 15 minutes). At 89K miles, your FI's may be worn out.

 

Have a FP gauge early next week. Be on hold til then. Does it hot or cold so I'm guessing too its not a fault with the O2. Thanx for the input.

 

gccsv, It would be better to start a new thread on your situation...IMO. It will take longer to dismantle the L98 fuel rail to pull and visually check for leaky injectors...though would be the best way to check for leaky injectors. Your plugs being gas soaked would point to leaky injectors. You will need the fuel pressure guage to further diagnose the problem. When the fuel system is primed and drops off suddenly, of the course of a few minutes, it would indicate a leak in the system. Different tests on the fuel system will point out what is leaking. You can also ohm the injectors hot and cold for more info...all should ohm around 16 each +/1 one...any less or any more will tell you the coils are not functioning properly and the signal sent to them is not accurately spraying the correct amount of fuel. You also have a Cold Start Injector (9th injector) which if ohmed will read around 4-6 ohms and is correct. Though the only way to tell if it is leaking is to pull the fuel rail.