Throttle lag issue

[damounh]

I got my E30 about a year ago and have been trying to fix most of the issues. Some oil is leaking from the cylinder head, so will need to take it off, smooth it out and replace the head gasket. But my problem right now happens is a noticeable lag in throttle response.

When pressing the throttle, it takes a while for the engine to rev up, when it finally does and throttle is released, the engine seems to struggle. Please take a look at the videos below, you might notice the sound of me pressing the throttle and the delay till the engine revs up.

Anyone have any ideas what could be the issue?

Warning, video might be a little loud
Engine cold start: https://www.youtube.com/watch?v=Yrk6nFaeJs0
Engine warmed up: https://www.youtube.com/watch?v=TmK6yYisT70

[jlevie]

Without hands on access to the car and running a set of diagnostic procedures I can't say what might be wrong. Below is a trouble shooting guide that should help. Start at the top and work to the bottom w/o skipping any steps. The odds are that by the time you are done the problem will be solved.

Idle, hard starting, and engine operation problems are most commonly caused by intake leaks and/or a sticky or defective Idle Control Valve (ICV). The only reliable method of locating intake leaks is to have a smoke test run on the intake and crank case and to test the brake booster with a gage and vacuum pump. The complete list of possible causes of an intake leak is:

Intake boot
Throttle body gasket
ICV hoses & connections
Brake booster, hoses, and connections
Crank case breather hose
Evaporative control hoses, valve, and expansion tank
Fuel pressure regulator & hose
Injector seals
Valve cover gaskets & bungs
Oil filler cap
Dip stick o-rings
Oil return tube o-rings
Pan gasket or oil level sensor gasket

While leaks in some of those can be found by inspection or by spraying carb cleaner on suspect areas, not finding leaks that way doesn't eliminate the possibility. Only a properly executed smoke test using the right equipment will really work. That means plugging the exhaust, replacing the AFM with a plug containing a nipple for smoke injection, and using a professional smoke machine that uses oil for long persistence smoke. Then you pressurize the intake for about 10 minutes at 2-4psi and watch for smoke. Note that if there is a major leak, it must be repaired and the test repeated.

Once the possibility of intake leaks is eliminated, the ICV needs to be removed and cleaned with carb cleaner until the vane inside moves freely. When the ignition is switched on you should be able to feel vibration from the ICV. If no vibration the ICV is bad, there's a problem with its wiring or connector, there's a problem with the TPS, or the DME (or Idle Control Module (ICM) on an ETA car).

For the DME (or ICM) to control idle, the idle switch in the TPS must work correctly. The switch should close when the throttle stop is 0.030-0.060" off the idle stop screw. Hearing a click doesn't mean the TPS is working. Check with a meter or test light by removing the back shell on connector and checking for the signals at the DME (or ICM) with the ignition on.

On an ETA car the cold start valve and it’s associated sensors must be functional.

The fuel system should be tested via the suite of tests in the Bentley manual as invalid rail pressure can be a contributor to idle and starting problems. A simple injector check is to pull the injectors, jumper the fuel relay to run the pump, and see if the injectors are leaking. You can also point the injectors into a towel, remove the coil wire, and crank the engine to see if all of the injectors appear to be spraying in a similar fashion. The best approach to possible injector problems is to have the injectors cleaned and flow tested. Since raw fuel can or will be released in these tests, have a fire extinguisher handy. I use RC Engineering for cleaning, rebuilding, and flow testing. I will always do the injectors on an an engine with 100k or more one it. A noid light will prove that each injector is seeing a firing signal.

While a bad check valve in the high pressure pump can result in longer than normal cranking, if the fuel system is working as it is supposed to the rail will reach normal pressure in a few turns of the engine. A weak pump, clogged filter or leaking FPR in conjunction with a failed check valve can result in longer cranking and/or idle problems.

The O2 sensor can be a contributor to idle and performance problems. The O2 sensor is a scheduled maintenance item with a useful life of no more than 100k. If the sensor has that mileage or more (or is of unknown age), replace it. A good O2 sensor is vital for proper operation of the engine. The DME uses that data to adjust fuel trim for stoic operation. An aged sensor typically indicates a leaner mixture than actually exists, which results in the engine running richer than it should.

The ignition system can be contributor. Unless recently replaced with OE or OEM parts, install a new distributor cap, rotor, and wires. Use only OE or OEM parts. The third party (cheaper) stuff is an iffy proposition. Use NGK ZGR5A plugs as they are the closest to the original plugs that are NLA. If in doubt, replace the ignition coil with a new OE or OEM part.

The AFM can be a contributor. If the vane doesn't move freely or the resistance track is worn the DME may be receiving invalid data from the AFM. And if someone has fiddled with the bypass air adjustment the DME may be unable to stabilize idle. The bypass air adjustment should only be adjusted per the procedure in the Bentley and with an exhaust gas analyzer. And even then everything else associated with engine management has to first be operating properly. If the AFM becomes a suspect, replacement with a good used unit is the best approach.

Improperly adjusted or malfunctioning valves will affect idle, starting, and operation. As can compression issues from ring or cylinder wear or from the valves. A valve adjustment is called for every 15k. A useful diagnostic is to run compression (dry and wet)and leak down tests on the engine. Those tests will tell if the rings and valves are in good condition and the leak down test will usually indicate whether it is a cylinder or valve problem.

Although not usually a problem, a bad DME temp sensor is a possibility. That generally won't cause an unstable idle, but can cause hard cold or hard hot starts and/or a rough idle. As can problems with the timing reference sensors. Although not commonly encountered, a bad harmonic balancer on an M20B25 or M30 engine will cause problems.

When all other possibilities have been eliminated and idle, starting problems. or performance problems persist, replacement of the DME, or if applicable the ICM, is indicated.

In many cases the cause of problems will be a combination of factors. So it is important to test and repair all of the possible causes.