Hi everyone, I need some help I haven't been able to solve on my own. My 95 is running so lean when it's cold it bogs and pops like a carb with a plugged jet!

I have 56k miles,
Opti spark has been replaced twice
plugs just replaced
MAF sensor cleaned twice
this is a 95 so I can't use a scanner on it (OBD1w/OBDII plug), but there are no check engine lights.
it is not ASR, I know what that feels like when I forget to turn it off for a burnout.
fuel filter replaced
fresh gas
clean airfilter (not K&N)

Have had this problem for a couple years but it is getting worse. Car runs fine/perfect when warmed up but for the first 10 minutes or so it is hard to rev, have to feather the pedal really easy to keep it from backfiring. It sounds and acts like it is lean, so one of the sensors I think is not giving correct signal when cold.

I assume it could be MAF, MAP, or ???

Any help is appreciated, I have scoured the forums but really don't see any other similar problems for this year/model combo.

Thanks in advance!!

 

Check Fuel Pressure under load.

You can use a scan tool on it. Has to be OBDI compatible and have an OBDII cable. My OTC Nemisys works just fine with the OBDII cable, it has both GM OBDI and OBDII software.

Try cleaning your MAF.

If you can get a scan tool on it, check out the 02's and see what they are reading.

A lot of this will depend on scan tool, you cannot see what the ECM is seeing any other way. Other than back probing connectors with DVOM, hassle, big time.

I've chased a similar problem on a buddies 94. He wanted some other things done, so I didn't spend too much time with it. I hope to hunt with it some more soon.

FYI, his car ran lean (intake popping on acceleration) until it hit Closed Loop, I could watch it be in Open Loop and run like crap, then BAM instantly into Closed and ran fine, just like someone flipped a switch.

 

Idle Air Control Valve? possibly stuck cold start injector? based on what you say about it only occurring when cold.

 

I have cleaned the maf with CRC MAF cleaner several times. Unfortunately there is no scan tool available for these, I have an OBDI/OBDII tool, it won't work. I have a Carsoft 7.4 with CAN and all duplexing cables - no go. Been to every parts store in town, they don't have one either. Have access to a GM Tech2, it won't work, dealerships tell me they got rid of their old Tech1's.

I didn't think these had a cold start injector, either way, I have no trouble starting it at all, and no problem with the idle, so I don't think it's the IAC. Has to be something that regulates the fuel when cold, I was thinking Coolant Temp Sensor? That is only a minor correction or a major? My temp guage works fine though.

Found this:
"ECT Engine Coolant Temperature sensor water pump on drivers side coolant sensor value is used to invoke warm-up fueling, Idle Air Control starting position, fast idle speed, closed loop parameter and idle spark functions"

It sounds like what you describe, popping until it hits closed loop. What exactly controls mixture during open loop? I don't think the O2 sensors come into play until warmed up in closed loop.

 

Have you tried reading codes by grounding out the ALDL plug -
http://www.corvettebuyers.com/c4vettes/codes.htm
or you can purchase Gordon Killebrews ALDL grounding plugs at Corvette Central or any other vendors to check fault codes. Good luck.

 

O sorry didn't notice it was a 95'. There are 2 coolant sensors, the one for the gauge is the sender and does not communicate with the ECM, You want to check the temp sensor it's worth taking a look at, I'm pretty sure that could cause the issues described, I had a similar problem with my truck and it ended up being a frayed wire to the temp sensor.

 

Coolant Temp Sensor first. Do not use a Borg-Warner, their temp per degree does not agree with FSM chart and I had trouble! Since the car seems to run fine after closed loop acquired, O-2 sensors are probably working. Change out MAP Sensor, this will make the car run rich under both open and closed loop. The ERG Valve is always a suspect with a pin hole leak in the diaphram, there again if ok in closed loop probably not a problem. Next culprit for bogging and misfires are the exhaust check valves. Remove and blow into them from the exhaust side, if leaking replace, can cause rich fuel in both open and closed loop. While checking O-2's check for exhaust leaks close to O-2 sensors, will also cause richness in both open and closed loop mode. Last but not least are vacuum leaks. Always a problem in both modes but worse in open loop. Check intake manafold gasket with engine idling and sprayying throttle body cleaner on gasket area. Engine will increase revs if leaking. One last potenetial problem is the Ignition Control Module. They age from heat and become intermittant, particularly when cold (before closed loop). Again do not replace with Borg-Warner. Use A C Delco replacement electrical parts in this system. I spent several months chasing intermittents in this system. I seem to have gotten all of them out with the replacement of B-W with A C Delco. Its the computer that doesn't react well with them. I also have a 1995, LT-1 Vin P. Good luck.

 

Thanks for the input, it is helping me diagnose and I am leaning toward the ECT sensor, did some research on those and how to check, I may just replace it and see if that cures it. Been researching these injection systems more and more, according to one site:

"When the engine is first started the system goes into open loop operation. In open loop, the ECM calculates the Air/Fuel ratio based on inputs from the coolant and MAF sensors. The system remains in open-loop until the following three conditions are met:
• Coolant temperature above 105°F.
• Oxygen sensor has varying output voltage (indicating it is hot enough to operate properly).
• A specific amount of time (preprogrammed into the MEM-CAL) has elapsed."


So it seems to me the only two items affecting the fuel ratio in open loop are MAF and ECT. Once in closed loop, the MAF is still used, along with O2 sensors, MAP sensor, and TPS, but since engine is hot, ECT no longer in the equation? It would make sense then that a ECT malfunction when cold would be low resistance, and when hot, either ignored or the correct low resistance value.

If the MAF were bad, it would affect fuel ratio even when hot.

I will let you know tonight what I find, assuming I can get to the ECT on the waterpump easily to test...

 

Well, here's an update. Tested the ECS and it shows it is good, but I did find a leak in the high pressure fuel line at the tank. I had been getting a puddle under the spare tire that I thought was just water draining, but it coincidentally got worse this week and got hard to start Tuesday. Turns out the line was leaking. fixed that and I am no longer seeing the lean condition when cold. Hopefully that was my problem all along.

Thanks for all your help!!

 

Yeah, that might cause it as well as bad gas mileage.
Given the age of these cars repalcement of the fuel line o-rings is something to consider.

My 93 got all new o-rings on the fuel system to include lines/rail/injectors, the whole works this spring. The only thing not new is the rubber lines off the fuel pump as they are molded into the steel lines themselves.

 

That is the line that was leaking, the rubber line going to the fuel pump on the outside of the tank. spliced in a piece of high pressure fuel injection hose with a brass connector and 2 clamps on each side. It's been warm enough that I can't tell if it is fixed or not, it only bogged when it was colder than like 45 or 50 deg. out.

 

Well, I thought it was fixed, but it has been in the low 30's this week and still have the same problem. I did notice that when the coolant temp hits 140 deg. it is normal, so only when <140 deg. is when it bogs. The coolant temp sensor is good, so I am now at a loss as to what to check next...