DTM5: What’s a Wideband Sensor?

by CFlo

The most expensive used BMW is also the cheapest used BMW. This is an axiom that I’ve heard repeated amongst enthusiasts, basically expressing the fact that any cheap BMW has to have something terminally wrong with it that will cost you a fortune to fix. In my case, the cheapest used BMW just happened to be an E34 M5 that fell into my lap. All of my well meaning, logically thinking friends and acquaintances warned me not to become that sucker who gets fooled by the cheap buy-in of a depreciated German sports sedan only to be crushed by mountains of parts invoices and repair bills. Well guess what? They were partially right, but oh so wrong. The dominant axiom that applies to this M5 is our favorite in the DT offices: There is no better money spent than the previous owner’s.

This car has been through the hands of 6 or 7 owners including myself (CFlo) and DT Editor-in-Chief Vince (temporary custodian). We really don’t have an accurate count, but what we do know is that the owner before our buddy DWood spent an inordinate amount of time, effort and cubic dollars to rebuild the monster S38B36 3.5 liter inline six. Strangely enough, even though he sold it to DWood over 7 years ago, his photobucket album of the project is still active. Sorting through the pictures chronologically, I can see the progress and how it was rebuilt completely; there are pictures of new bearings, seals, the bare block back from machining, the custom 11:1 forged pistons from CP, the cleaned and reassembled head and throttle bodies, and the awesome bundle of snakes tubular stainless exhaust manifold that came stock from the Garching bei München ///M assembly plant back in 1993. But every time I peruse the album, all I can think is: “how much did this guy spend?”

I can only assume this was related to the engine rebuild. Image from P-PO’s album.

After dropping what I can only assume was a low amount on the purchase for an otherwise solid car with a blown head gasket (maybe $4000?) I’d wager the engine & trans rebuild tally was close to $8k. Add in the Dinan suspension kit (dampers and lowering springs – $1k), stainless exhaust system ($1k?) the NOS-NIB AC Schnitzer Type II wheels from the Fatherland (maybe $2k?) a decent stereo system (another $1k?)….and the total spend might have been around $17,000. Which – I happen to know for a fact – is exactly what DWood paid this guy to make it his.

The fresh shortblock, waiting to get some head. Image from P-PO’s album.

Seems like a case of the builder recouping most/all of his costs, when the car should have been worth maybe $12k tops at the time (despite the spend tally). Our buddy DWood is famous for overpaying however, so it all makes sense to us here in the DT double-wide office. What DWood got for his money was a fresh powertrain in a driver-quality (but handsome and presentable) E34 chassis. What he didn’t get was a fully sorted machine. After the glamour of the engine build is over few home mechanics are keen to spend the time vetting out all the little issues that are bound to arise after reassembling a vehicle. Identifying, diagnosing, and rectifying squeaks, leaks, and pings is the dark side of any car build – lots of time and frustration with very little to show for your efforts – until you finally get it over with.

One of the alleged NOS AC Schnitzer wheels being unpacked. Image from P-PO’s album.

The main thing that needed attention was the effect of the porting, polishing, flowing, etc. on the engine’s air/fuel ratio. Included with the car when DWood took ownership was a generic chip tune loaded into the factory Bosch Motronic DME (aka ECU). He soon discovered that the top end flowed quite a bit more than stock, and as such leaned the ever loving crap out of the fuel mixture and sent the head gasket packing via a constant stream of detonation. Pinging is seldom benign, but this wasn’t terminal; it simply took its toll and eventually the uncontrolled combustion “events” ate a hole in the head gasket.

Freshly rebuilt throttle bodies – claimed to be ported, yo. Image from P-PO’s album.

So of course, DWood threw money at the problem to make it go away. I’m just ribbing a friend here; he did a good job tearing down the top end, replacing the head gasket, and having the head resurfaced and rebuilt. The only problem was the shop he entrusted with the final work – a local So-Cal BMW tuning outfit that has since gone the way of the dodo, allegedly due to sloppy work and irate customers demanding refunds. This outfit sourced another generic chip tune (this time from jolly old England – oooh!), dyno’d the car again, but couldn’t figure out why the air/fuel mixture kept going lean above 4000 rpm at full throttle.

One thing the builder thankfully didn’t change – the magnificent equal length stainless factory headers. Image from P-PO’s album.

I don’t have all of his receipts yet but I’d wager DWood spent somewhere close to $10k re-rebuilding the top end and suspension. So he had a total of $27,000 invested. Of course no one in their right mind would pay $27k for this car at the moment – and E34 M5 prices have actually depreciated since 2007. But it is a great illustration of how costs can add up to outweigh a vehicle’s value in a ridiculous way. Another factor is that DWood is on semi-permanent assignment in China, and had been storing the M5 in his parents’ garage but could no longer do so and needed to sell it fast. On one of his trips back to the States he tried to get the lean issue rectified, but the incompetent tuner shop never was able to properly diagnose problem before folding up – so guess who got to take on that fun job – right, yours truly.

The results of DWood’s rebuilding and tuning efforts as measured by a Dynojet inertia dyno. Air/fuel ratio isn’t shown here, but presumably it was logged.

One thing the “challenged” tuner shop tried was plumbing compressed air into the fuel pressure regulator manifold reference line during a dyno pull. By doing so they were able to artificially boost the air/fuel ratio into a safer, richer regime. But take away the compressed air = back to dangerously lean. So we (DWood, Vince, and I) had to baby the poor S38 from then on, keeping away from full throttle above 4000 rpm, which is a downright shame and just depressing. This engine begs to be revved to redline at WOT. The damned lean condition had to be fixed, but how? Maybe with the help of a real tuner shop, some common sense, a better dynamometer, and a wideband O2 sensor?

Stay tuned for the next installment, when the DTM5 hits the dyno…again.

Just for your entertainment, here’s the text of original Bimmerforums.com for sale ad from March 2007, when DWood bought the car:

1993 BMW E34 M5

Last year made for USA Market Hand built in BMW Motorsport, Germany

Black on Black leather

Sport Seats

Every option

Full Records since New (100’s of reciepts)

150,xxx Miles

No Accidents Ever

Ice cold A/C

Dinan Stage 2 SLS Delete Suspension

Dinan EProm

Dinan Cam gear

Tri Flow SS Cat Back exhaust system 

AC Schnitzer Type II 17×8.5(f)17×9.5(r) Wheels (1995 Dead stock shipped from Germany)

New Toyo Proxes 4 225/40 (f) 255/40 (r) Tires

Brembo Drilled Front rotors

Rare nardi 14″ Steering wheel

Bimmian Aluminum Shift knob 

Original BMW Motorsport Mirrors

Original BMW Motorsport Door Handles

Ellipsoid Headlights from later 750iL

Hella Euro Tail Lights

Hella Euro Front corners

Alpine 9856 Headunit w/ipod connect

JL Audio 250/1 & 300/4 amplifiers invisibly mounted on rear deck

Bavarian Soundworks front 2.5 inch midranges

MB Quart front+rear components mounted in stock locations

Full Engine rebuild performed my myself (I am a auto engineer/mechanic) @143,xxx Miles

Engine dynoed @301 to Wheels! 286 Torque

Every part that wasnt perfect was replaced from oil pan up!

CP 11:1 CR Forged Aluminum Pistons 94mm

Fully Blurprinted/Balanced Crankshaft,Rods,Camshafts

Cylinder head by Steve @ Hollywood Racing:

Flowed and ported 

Full 3 angle valve job

All new valve springs,bronze valve guides,seals,keepers

VAC Motorsports MLS headgasket

BMW M3 Updated Chain tensioner

New Oil Pump & Drive Gear

94mm Bore bottom end, all new rod and crank bearings

New Behr radiator,water pump,thermostat,overflow tank & all coolant lines

New oil cooler and oil cooler lines

New Motor mounts

All components have been hot tanked,media blasted,and painted this a beautiful and clean engine

Fully rebuilt intake manifolds (individual throttle bodies):

Disassembled and media blasted better than new

all new gaskets,seals,hardware has been renewed or replaced

Fully synchronized and flow perfectly

Rebuilt Fuel injectors flow within 0.9% of each other with all new internals-flow bench tested to 9000 RPM

New high pressure fuel supply lines


Fully rebuild by the only experts on the west coast for Getrag 280’s

New synchros from 1-3

New 5th gear was installed

Red Line Tranny Fluid

UUC Motorwerkes street/race tranny mounts

UUC Motorwerkes enforcer supports

Short shifter kit

Fully Rebuilt Shifter linkage 


Rebuilt/balanced Driveshaft (new guibos)


Is original and never repainted, shows some chips and a couple of dings, expected from a 13 year old car. Overall Very good.

No rust ever! very solid garaged California BMW.

Check out all the Daily Turismo Project Car posts here for both the DTM5 and our old Volvo 242.