please help my 1989 300se is surging when I am at stop lights. Have to hold my foot on brake and pat the gas to keep it runing, can anyone help. The car has been tune up but I am still having this problem.

 

Kfulks, I'm no pro mechanic & don't know your year car but that sounds just like what my S320 did when the Fuel pressure regulator went bad. If it is it’s an easy fix and not to expensive for the Part. Mine was on the top right front of the engine. I was told to pull the vac line off the regulator and check it for gas or the smell of gas. If its wet with gas or swells like gas it is bad.

 

In reality what "could" be happening is the electrical component inside the MAF sensor becomes contaminated by the impurities in the air. These containments imbed themselves to electrical component that senses the air flow causing the sensor to be slightly off. Note that the filament does heat up upon ignition to burn off any residue, however it is a place to start.

Engine surge is "primarily" caused by a engine management failure. Sensors that monitor emissions and performance are communicating with the main PCM controlling fuel mixture, timing and emissions management systems. Failure to do so can cause a surge while driving.

Example:1 If sensor value is out of range the main PCM will "hunt" for value therefore causing a surge condition.

Example:2 If a vacuum leak is present in the intake manifold or intake boot is torn this will cause one or more sensors to malfunction because they are out of operating range.

Basic Check

a. Testing for DTS's (Diagnostic Trouble Codes) for this you need a PCM Scanner, this is a small hand held electronic device that plugs in to your cars OB2 (OBD11) diagnostic connector. This would be most helpful, if you have access to one. This device can tell you the trouble code that was stored in your cars Main PCM. (Power Train Control Module). Read Trouble Code Chart

b. Testing fuel pressure. Test for proper fuel pressure with gauge on fuel rail. If no or little fuel pressure and there is power present the fuel pump, the pump needs replacing.

c. Check for vacuum leaks, listen for obvious leaks. Inspect air intake boot for tears, this will foul the MAF (Mass Air Flow) sensor into thinking the engine air intake value is less than what it is. Or a IAC (Idle Air Control) bypass hose can develop tears/holes in the middle of the hose so inspect thoroughly.

d. Check for plugged exhaust system. Excessive back pressure can cause exhaust not to flow causing engines to surge and buck and lose power. "Some" exhaust systems need to be replaced between 100,000 and 120,000 miles.

e. Test for weak ignition spark. Remove spark plug wire. Insert an extra spark plug into end of wire, attach spark plug to ground then crank engine. You should observe blue spark between spark plug gap. If a yellow spark is observed, your ignition coil is weak and needs to be replaced. If your spark is not consistent you may need a ignition module.

Without seeing your vehicle (codes) for a visual inspection it is difficult to diagnose. Hope this points you in the right direction. Note: Fuel Filter should be replaced (cheap fix) every 15k.