Really Poor Mileage M50 Swap

[Kerrath]

Well, my mileage isn't so fixed after all.

I typically get about 20-22mpg across a tank, with one spike of 24mpg when I did a large amount of freeway driving. It's better than what I was getting in the volvo, but not as good as it should be.

I used lifeiskaos' compression tester today and determined that all my cylinders are healthy.

I have a 3" 90* silicone intake boot instead of the stock one, and other than that, no modifications.

I have had a light ticking once warm that does not sound like it's coming from the head and is too high pitched to be any kind of knock or a sticky lifter. It's also relatively quiet and slower. It varies directly with RPM, making me think it might be a dying injector. I replaced the least good looking of my fuel injectors today, and they're all tested to be in the proper ohm spec. I need to drive it more before I find out if the tick is gone.

I installed a 4" spacer before the MAF for a full tank of gas because I once read that the turbulence caused by a cone filter right on the MAF causes power loss and lessened efficiency, but I had little or no improvement as a result. Definitely not enough to warrant the hassle. I also only get the all clear code when I do a stomp test.

One thought that accounts for some of the efficiency loss is that I have a 2.93 differential and about 1/2 of my driving is done on surface streets. On the freeway, I do not use fifth gear because I need to be going like 90 to make it any kind of useful. I do not think that that differential accounts for all that much fuel loss, however. I will be getting a 3.73 LSD soon, so that should help some, but still.

So, with some new information, what do you guys think could be up/at least worth checking out? Could that diff really cause such a significant decline in efficiency?

[e30serg]

Are you 100% sure all your sensors are good? I remember my gas mileage dropping when I had a bad O2 sensor in my m20. What about getting a 3.25LSD from an e28?

[Kerrath]

I am like 85% sure all my sensors were good. The only ones I can think of right now that are untested are the IAT and CTS. I wrote down the specs for those last night so I can test them later. The MAF was bad (stomp test) and I put a replacement on and that went away, so it should be good.

The O2 sensor is quite new, but if by some fluke it went bad, I know the stomp test sucks at telling me. I had to get it diagnosed with an OBD reader last time.

3.73 LSD on the way. I'd've gotten a 3.25 LSD if I'd've seen one. People tend to take them, or just take the tags from them at the yards. >:[

[hoveringuy]

You should be getting around 27mpg on the highway. I'm getting that in my 3.0!

One thing I have learned in the last 9 months is that the 413 ECU is extremely good at compensating for even the most screwed-up sensor. If you have a bad sensor it will work very hard to trim your mixtures to 14.7. I have seen mine do that with big vacuum leaks and weird sensors. I can watch it happen on the wideband.

Only thing I can think of is that the motor isn't getting warm enough like your thermostat strap broke and it's permanently open, causing the ECU to want to make it rich or your exhaust is clogged and the motor is working harder to pump the exhaust out. Or you're running 90 weight motor oil?

You should drive with a wideband for a few days, it would answer a lot of your questions.

[Kerrath]

You should be getting around 27mpg on the highway. I'm getting that in my 3.0!

One thing I have learned in the last 9 months is that the 413 ECU is extremely good at compensating for even the most screwed-up sensor. If you have a bad sensor it will work very hard to trim your mixtures to 14.7. I have seen mine do that with big vacuum leaks and weird sensors. I can watch it happen on the wideband.

Only thing I can think of is that the motor isn't getting warm enough like your thermostat strap broke and it's permanently open, causing the ECU to want to make it rich or your exhaust is clogged and the motor is working harder to pump the exhaust out. Or you're running 90 weight motor oil?

You should drive with a wideband for a few days, it would answer a lot of your questions.

I'm running an 88* thermostat. The old one was stuck open so I replaced it. I run barely below 1/2 on the late model temp gauge. Before that I ran in the blue. The only gain I noticed for running warmer was a slightly smoother idle. If I gained efficiency, it was very small, like .5mpg, and that's hard to tell since they always switch around the gas mixes.

Since mine's Non vanos, I've got either the 402 or 405 ECU, can't remember which right now.

My friend may still have his wideband, but isn't it gonna have screwed up readings, since the car'll be running in limp home mode without an O2 sensor?

[dirty30]

Do you have ebay headers? I know a few swapped cars were running poor due to the placement of the O2 sensor, if its too far back it gets cool to quickly and throws a CEL. Just a thought...

[Kerrath]

Do you have ebay headers? I know a few swapped cars were running poor due to the placement of the O2 sensor, if its too far back it gets cool to quickly and throws a CEL. Just a thought...

Yep, I've got the ebay headers. No CEL and the stomp test shows clean. The O2 sensors are heated besides, so it's hot. It's about 6" behind the headers on mine. It doesn't seem like it's running poorly, just inefficiently. I have a low, but steady idle (~520 rpm) and the engine doesn't hesitate or stutter anywhere else either.

[Kerrath]

Hypothetically speaking, if the O2 relay is failed, and the O2 sensor is too far back to be hot enough from exhaust, how can I test if it's hot enough where it is? I don't trust the CEL when it comes to O2 sensors because the ECU knew the sensor was bad, but it didn't show in the stomp test.

[Kerrath]

So I drove about 50 miles last night and it seems like my ticking has gone away. Or it got quieter and more of them, so I'm quite certain it is the injectors now.

However, now when I slow down without the clutch in, my idle oscillates between 450 and 550 and occasionally gets bad enough that it thinks its stalling. At first I thought it was brake associated, but it happens whether I press brakes or not. It idles smoothly once a bit warm.

I unplugged the fuel rail completely to install the injectors. I removed the PCV hose from the valve cover by accident, but it's put back. I replaced my silicone intake boot with the stock one because I wanted to repair a crack in the silicone I put around the fittings. To the best of my knowledge and eyesight, everything is plugged back in now, so I do not know what could be causing this business.

[dirty30]

I have the same problem with my car.... Runs smooth if driven consistently but idle gets rough like a v8 with a hot cam. I know for a fact it is because of the placement of my 02... The sensor is a new unit from the dealer and the only code I have present is an 02 fault. Keep in mind vacuum leaks are also known to cause maf/ 02/ mixture faults or sometimes no codes at all in older motronic systems. If you have access to a smoke machine maybe give it a shot.