I had the same problem with the same car and engine! Since you said no codes were present I think you have a vacuum problem. A vacuum leak directly affects two critical functions to the engine. #1 the fuel pressure regulator, #2 the map sensor. Both items can cause misfire symptoms. Several other items are also affected but they are not related to your engine problems. Up near or at the EGR valve you will see a vacuum main plug-in. One hose goes to the EGR valve, one hose goes to the fuel pressure regulator, one goes to the fuel vapor
canister, and one goes to feed the vehicle ac/heater vacuum needs, the cruise control needs and...the map sensor!!! All the items are affected by a vacuum leak at the same time. I found several cracked "Black Plastic Tubes" were cracked due to old age. Some of the rubber hoses were also show age as well. I looled for this problem for a very very long time. I discovered it by chance. The problem is several of the tubes cross under the upper intake manifold. If you already replaced the fuel injectors you may have notice the tubes and hoses going the vapor canister and the tube and hose going to fuel pressure regulator and yet another tube going to the back of the intake towards the firewall. This particular black tube is very important to check out. Before this tube makes it's exit from under the intake...there is a TEE fitting there. As this TEE is setup one part ot the TEE goes back under the intake manifold and serves, some other unknown that I never noticed with the intake was off and can only be accessed with the intake is off!!!. The other part of the TEE exits from below the intake to to another TEE to feed the MAP sensor and the the ac/heater box vacuum controls as well as the vacuum needs of the cruise control. What ever the item is under the intake is was wheer the first hint of the problem started. You can run a by-pass tube over the top to connect the MAP sensor back up and the vacuum tube going to the firewall to feed the cruise control abd ac/heater needs. You can replace the fuel pressure regulator tube and hose. I had notice the cracked tube to the vapor canister while the intake was off and replaced it at that time. I have the by-pass still connected and the car is running great. Symtoms Noticed: AC/Heater vents would slowly switch front AC vents to defroster under heavy engine loads like mild hill climbing. Cruse control would slow loose the ability to maintain selected set speed. Engine would hesitate from a stop and sputter upon acceleration and was somewhat sluggish and would also sputter under light load demands for cruising and slugging response for more power. Engine would respod quickly to high power demand only after pedel was pushed down to 3/4 throttle only, upon returning to normal throttle needs the engine would again sputter like it was misfiring. Hope this helps, you might now notice if all these symtoms are occurring in your vehicle. My car was never actually misfiring, it was due to a lack of fuel demand because the fuel perssure regulator has NO vacuum and was staying closed like it would at idle!!! Good luck.
May 9, 2026 at 4:43 PM
