1989 Ford E-Series Van • 297,011 miles

Have a Ford E150 Club Wagon 351 Engine 5.8 L V8 VIN: H. At 45 mph hesitates or it fails to accelerated or will go faster then suddenly drops speed And then holds at 45 mph with equal or more gas application but won't increase speed unless it back fires. Once it back fires instantly it'll speed up. However, in short time drops speed again. Has lost acceleration and lowered mph running between 25-35 mph for 7 miles then back fired and accelerated after up to 45 mph. These are the repairs: fuel injectors power flushed, throttle body and idle air control cleaned. Replaced: fuel filter and air filter, spark plugs, spark plug wires, distributor cap and rotor, O2 sensor, ignition module, fuel injected regulator, fuel pump, MAP Sensor, ERG Valve Selenoid, muffler system and catalytic converter. Back firing has been much less upon new ignition module. Once the MAP sensor and EGR selenoid were replaced, cleared codes and then the fuel pump showed lower pressure and was replaced. Yet, drove several miles and then the same loss of acceleration and power happened with engine light coming on for the first time. Previously, engine light did not come on. Waiting to hear what codes show from the engine light. Throttle body sensor has been checked, honed ok. Any ideas on the cause? Thanks for your time and consideration in this matter! Zwheelin
January 23, 2012.

It sounds like you have replaced just about everything there is. Have you checked for vacuum leaks? Are you sure the fuel pressure is maintaining?

Yes, vacuum leaks were checked, found a couple and were said to be repaired. He said he hooked up the fuel pump and drove it to see that it held correct pressure and said it maintained at 30 as he believed was accurate. Any idea why it is at 45 mph it has the trouble? Consistent at that speed the issue starts. Thanks!

Jan 23, 2012.
Speed and RPMs are the same at 45. That leads me to check the throttle position sensor. Also, make sure the exhaust system isn't partially plugged. The catalytic converter could be going bad too.

Throttle position sensor checked out. He said "it honed fine" but I've read that it could have intermittent trouble. Mechanic said he believed the sensors were an all or nothing part. Is that a solid fact? Catalytic converter was said to have been checked and all new pipes and muffler put on, But a different mechanic did the test on catalytic converter and replaced muffler system. This mechanic said he couldn't figure out why the muffler was excessively hot. Can a partial plug in the muffler or catalytic converter be causing the muffler being too hot by not getting air through it properly? Thank You!

Jan 24, 2012.
Electronic parts are known to go bad intermittently.