Using OBD To Solve Post-Rebuild Drivability Problems
By Norm Brandes
I work with a lot of shops in my area, so it’s not unusual for people to call me asking for help in solving various drivability problems that may occur in OBD II vehicles after the engine machine work and installation have been done. I’m usually given a symptom such as hard starting, rough idle or no power. Once in a while, they give me some trouble codes or scan tool information, and on rare occasions, even some fuel pressure, voltage drop and lab scope data.
But to get a complete picture of what’s really going on, I need three types of supporting information: a complete description of the symptom (what exactly is happening and under what conditions), scan tool data (sensor inputs, current codes, pending codes, history codes, etc., preferably taken while the problem is occurring), and physical test data (compression, vacuum, voltage readings, etc.). Without this information, it’s hard to second-guess what might be causing a problem.
A recent example that comes to mind is a call I got from a West Coast shop that had recently replaced a customer’s engine. The customer complained that the engine lacked power. The engine would run fine up to three-quarter throttle, then fall flat on power.
The guy at the shop seemed to be knowledgeable and familiar with diagnosing electronic fuel injection. He had scanned for codes and found nothing. He had also checked fuel pressure and said it was OK. He had also used a lab scope to check circuit integrity and found no problems there either. Compression was normal; there was no indication of an exhaust restriction, and the tailpipe emission numbers were all good. The guy was stumped and needed some direction on what to try next.
When I asked him if the vehicle had been driven, he assured me that it had – several times, he said – but there were no codes set. So we focused on the physical tests that had been done up to that point.
One thing we had to determine was if the engine control system was responding correctly. I asked him to check the throttle position sensor to make sure it was reading all the way to wide-open throttle (it was). I asked him to look for a timing change occurring on the crankshaft and through the scan tool as rpm changed (it was). I asked him to check cranking compression and dynamic compression (the numbers were both good). I asked him to check fuel pressure at wide-open throttle, and to check injector resistance, current flow and injector volumes against a known good injector. He said everything checked out OK. I also asked him if any maintenance items had been replaced since the engine was rebuilt, such as the O2 sensors, fuel filter or air filter. There were no changes here that would affect performance.
He had checked everything I asked him to check and had done everything right as far as the engine assembly and installation was concerned. Yet, the engine lacked high-speed power. Something obviously wasn’t right, but what had we overlooked?
I had gone through quite a laundry list of things to check that might be causing the loss of power at wide open throttle, but we had turned up nothing. It’s important to check all of these things because if you don’t, you can end up replacing good components, unnecessarily aggravating your customer and yourself.
After asking repeated questions about how the car responded to different driving conditions, I discovered that the car was only being driven for short trips – mostly at wide open throttle – which is not the type of driving conditions that OBD II needs to detect faults.
Before OBD II will set a code, all of its various system monitors must be ready. Some monitors run continuously and will detect problems under any type of driving condition. But, other monitors are not continuous and only run their system check after certain driving conditions have been met. The exact requirements vary from one make of vehicle to another, but as a rule require a variety of different operating conditions such as:
- Cold starts;
- Low speed driving (below 30 mph);
- Cruising speeds with light acceleration;
- Hard acceleration; and
- Maintaining a road speed of 50 to 70 mph with very little throttle movement for 10 minutes or more.
If all of these have not taken place, some of the OBD II readiness flags may not be set, and the system will not detect what’s causing a problem. So, I told the shop to take the car out for a lengthy joy ride. I told him to make sure the vehicle was driven under these different conditions and see what happens.
Guess what? After driving the car as instructed, the "Check Engine" light came on. Once all the system monitor readiness flags were set, OBD II detected a problem and set several codes that indicated a lean fuel condition. We now had some direction on what was going on and what might be causing the problem.
The fuel pressure test I had asked him about earlier had been done at idle, so it did not reveal the weak fuel pump that was causing a drop in pressure at wide-open throttle. Replacing the fuel pump cured the customer’s problem, and everybody was happy.
The shop could have saved itself a lot of time and grief had they simply allowed OBD II to do its thing. OBD II is an intelligent diagnostic tool that can save you hours of testing by checking itself. But it can’t do it instantly. The vehicle has to be driven long enough under a variety of different operating conditions to set all of the OBD II system monitors. Then and only then can OBD II identify problems that may be affecting engine performance.
The status of the monitor readiness flags can be easily checked with a scan tool. If all the monitors have been set, any problems that would normally set a code should set a code. And if the monitors are not ready? Then the vehicle needs to be driven until they are set.
So the next time you encounter a "no code" drivability problem with an OBD II vehicle, make sure the vehicle is driven long enough and under the right conditions to set all the OBD II monitors. Given a chance, OBD II will find most of those problems for you – but it takes time to set the codes.