Vehicle
1984 through 1995 Volvo 4-cylinder Turbo (740, 760, and 940 with B230 FT engine).

Symptom/Problem
Lots of black smoke on acceleration while in a boost condition. On acceleration under turbo boost, the car will lose power and may have black smoke coming out of the tailpipe.

Background
This problem may not occur every time it accelerates, but will usually be worse on harder acceleration. The car will run just fine under all other conditions. Initial testing of engine and tuning basics will show everything adjusted and functioning just fine - no problems. Cars manufactured in 1990 and later may output a generic fuel mixture fault code such as 113 or 231, but the MIL will usually not be activated.

To understand the cause of this strange behavior, it is first necessary to understand how the fuel mixture control system works. The Bosch LH-Jetronic system (LH 2.1 in 1984, LH 2.2 1985-1989, and LH 2.4 in 1990 and later cars) uses a hot-wire mass-airflow sensor to meter intake air. This sensor is positioned in the intake hose of the turbocharger so that air is metered as it is drawn into the turbo. This means that any loss of air after the turbo will not register at the LH-Jetronic control unit. It will still be adjusting injector duration on the basis of information from the mass-airflow sensor, on the inlet side of the turbo.

Test and Fix
On these cars, it is necessary to carefully check for intake air leaks under pressure between the turbo outlet and the engine. Very often, such leaks will be the result of cracked intake boots or stripped hose clamps that will not show any leakage with the usual tests using propane or brake cleaner with the engine running at idle, with vacuum on the intake system. The system must be pressurized to show leakage. The greater the degree of leakage, the richer the system will go under turbo boost.

One additional note regarding this problem: Testing with the greatest care may not show any leaks, or the leaks may be repaired and the problem will be unchanged. Another failure that can give exactly the same malfunction symptoms is a fuel pressure regulator that sticks shut with pressure on the vacuum line from the intake manifold, increasing the pressure to the injectors to full pump pressure, which is near 100 psi. To determine if the pressure regulator is malfunctioning, monitor fuel pressure under boost.

David Tidaback is an Identifix European specialist. He is BMW, ASE master and L1 certified. He is also certified in Volvo engine and fuel injection and Bosch BSC 1.

Maybe "Cold" Light on Subaru Would Be Answer to Its Stumbling

By Todd Erickson

Technology Reversion
As I climbed into my '65 truck and pulled out the choke knob to get it started on this brisk evening, I was still pondering the hotline call on the 1995 Subaru Legacy with the single stumble after starting cold.

The "Cold" lamp on my truck was shining brightly and I knew I had to wait for a minute before attempting to drive it or it was sure to sneeze and die.

Dealing with cold driveability problems in the '60s, when my truck was new, must have been a lot different. I can just about hear how that hotline call would have gone back then:

"So, you say your 1965 truck stumbles when it's cold out?"
"Yes."
"Does the 'Cold' lamp come on?"
"Yes."
"Well, let it warm up before you attempt to drive it. You have to wait for the light to go out. If you don't, it won't run right."

Back then nobody even attempted to drive their cars before the engine totally warmed up, let alone within seconds of starting them. I wonder if we could install a "Cold" lamp on that Legacy?

Todd Erickson is an IDENTIFIX Asian specialist. Erickson is an Accredited Automotive Manager (AAM) with 16 years of diagnostic experience. He is a certified Subaru senior master technician, as well as ASE master certified, L1 and alternate fuels.

This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time. For more information on IDENTIFIX, call (800) 288-6210, 8 a.m. - 6 p.m. Central Time. www.identifix.com. © 2008 IDENTIFIX. All Rights Reserved. © 2000 IDENTIFIX. All Rights Reserved.

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