Just got done replacing the intake manifold on my 91 L98 coupe with 101k miles to get rid of a large oil leak present at the back intake/block interface. Prior to doing this work I had put a new set of Bosch 24lb injectors from FIC into the engine to get it back up and running. The oil leak appears to be gone, but am having problems after I drive it for 15-25 miles that the SES light will come on. I have read out a code 44 which indicates lean exhaust. This will get set when driving at 60 mph with the cruise set. If I put my foot into the throttle a bit just to bump it up to runnng around 65 mph for 30-60 seconds, the SES will go away. I have also noticed that there is a bit of hesitation in the throttle response when trying to give it 10-20% throttle upon starting from a dead stop. On top of the above when I reset the average fuel economy it over the last 100 miles of 25% city and 75% highway driving is showing around 13.5 mpg. Prior to getting the engine back up and running I put a new Bosch oxygen sensor in the vehicle.

I am thinking that I have a leak in the intake system somewhere that is causing at least one side of the engine to run lean and tripping the code 44. I do not know if the engine is locked out of running in close loop with the code 44 in memory without the SES lit or not. I have a request in to our service group at work that has a Tech 1 to borrow it for the weekend to monitor the o2 sensor voltage, closed/open loop operation, and look at where the block learn values are at.

Not ambitious this evening to go out and work in the 100 degree weather to try to find if I have an intake manifold leak. I have heard of others doing this with a propane torch set to provide minimal propane flow and go around all intake interface points to see if the idle rises. Has anyone had success in doing this? My fear is the leak is down in the intake to head interface and not in the tuned port runners as that would be supported by Murphy's law.

Any insight or recommendations are appreciated on how to efficiently troubleshoot this problem.

 

Filled up this morning and calculated getting around 20 mpg for the last three days versus the 13.5 being shown by the instrument panel. This has me less concerned about the fuel economy, but the SES light did come on again this morning after driving around 20 miles. When I filled up with fuel and restarted the vehicle, the SES did turn back off. So still dealing with an intermittent code 44, but with less concern about the fuel economy. Right now I will assume the use of the Bosch injectors instead of the originals is what is throwing off the fuel economy calculations.

 

Monitoring your O2 with the Tech 1 should tell you a lot. Hopefully I have my thinking cap on strait but a code 44 means that the O2 voltage was to low thus in increased the integrators and block learn values above 128 to increase the injector's pulse widths to increase the fuel to compensate for the lean condition. I don't think it's your injectors because they would cause the opposite problem because they are slightly larger then stock. Did you check your fuel pressure, I've had foreign material get into the regulator's valve after a teardown causing to low of a pressure that would cause your problem, should be around 42 psi with vacuum disconnected. Also, just because a part is new does not mean that it's good. Maybe we are over thinking this problem before you get the Tech 1.

 

I will take a look at the fuel pressure tonight. Before I put the plenum all back on I checked the fuel pressure to make sure the two main seals to the supply and return lines for the tank were not leaking. I seem to recall more around 35 PSI versus knowing I was getting more around 40 or so prior to taking everything apart. Was a little perplexed with it being lower, but may be will worth addressing the fuel pressure regulator.

I know there is a new sending unit in the tank. I had problems of low fuel pressure with 3/8 of a tank of gas and had to pull the car a 1/4 mile home to work on it. Pulled out the fuel sending unit to replace the pump and then looked down in the tank to find out it was dry. Had a problem with the sending unit telling me I had fuel when I didn't, but replaced the pump due to already having everything pulled out.

Previous owner claimed her mechanic had replaced the fuel filter in their attempt to get the vehicle running before I bought it.

 

I had put 24lbs. in my 86. Iwas told I had to get the ecm changed. I did and it runs great. I only get 16 mpg in city. I have done a lot of other things to the engine.

 

When I chose to go with the 24lb injectors I was not given a heads up by FIC that any tuning of the ECM was necessary. I will give them a call on Monday to ask them that directly after I check the MAP sensor and the fuel pressure regulator here over the weekend.

Who did you have modify the calibrations in the ECM to adjust for the new injectors?

 

..... An increase in manifold pressure - vacuum leak - will be detected by the MAP sensor ... which will then signal the ECM to command more fuel to compensate for the higher - assumed - load ... recheck all your vacuum line connections ......... Did you reset the minimum idle and TPS ?

 

I did not reset the minimum idle and the TPS. Is this procedure in the factory shop manual? If so, what section would I find it to be able to perform it?

I did check vacuum connections once, but will do so again as one could have been missed. (I had found one when checking the first time that was a T connection by the driver's fender that got opened up that was not even in the area I was working).

 

As eluded to, Lean means it's dumping fuel, probably fuel it doesn't need, most probably because air is flowing past the O2 that didn't come through the Throttle Body. Given that you replaced the Intake Gaskets, sniff around the Intake, Plenum and Runners with some propane. If the idle speed increases, you've got a leak. Otherwise, a vacuum line that wasn't reconnected or is broken is also a good place to look. Leave minimum idle and your TPS alone. They have nothing to do with a 44, nor will they change from any Factory Setting. Neither is there any need to reprogram for your Injector Change. Your ECM is working as it should as is the O2 which is being tricked into thinking there's a lean condition.

 

I am leaning toward what is recommended here that I have a leek somewhere after having done the intake replacement, either around the plenum or around the intake. Sure hope it is not around the intake as that will be much harder to get to and repair. Just took the car out and drove it for 20 miles without the code being set, so it is definitely intermittent. I did check the fuel pressure and it was going up to 45 psi after turning on the fuel pump and staying in the 40 psi range as I increased throttle.

 

I tried going around the intake manifold interfaces, the plenum runners and the throttle body with propane using a hose added to the end of my propane torch and the air holes taped off to make it pure propane. I was unable to locate anyplace that the idle speed would change indicating a vacuum leak. I am still thinking a vacuum leak is my issue, just not had luck finding it yet.

 

that's not how I interpret the 91 fsm. 44 says "lean exhaust condition."

lean means the sensor sees lean, not a rich condition that the ecm is able to compensate for by dumping more fuel. I may be reading more into it. but it -fsm- suggests that injectors may be unable to flow enough fuel, thus a lean condition.

or, it is leaned by ecm because it believes the sensor was seeing rich, or there is an exhaust leak just ahead of the sensor.

am I confused?

 

I don't know if you're confused, but if you read the conditions to look for in the Trouble Tree, you'll stumble across a solution. Anyway, yes a Lean O2 - when the air source is anything other than the throttlebody makes it rich (unless it's a bad O2). Fuel Delivery is a function of TPS, CTS, load (MAP on this car) and whatever that O2 signal says it should be. Air leaks introduce another source of air (or a variable it wasn't tuned for) and that mucks up everything else, so it gets too much fuel. You don't see it on a scan, but sometimes you'll smell it and it will definitely show up on a smog test and make it fail. That's why the the CE Light gets nice and bright and in the future, it will be probably be transmitted wirelessly to some DMV and if you don't fix it, they'll turn the key off until you do.