My daughter has a 1996 Toyota Corolla that she bought last year from a personal seller who kept impeccable records on everything that was ever done to the car for maintenence, etc. My mechanic ok’d the buy. For the past few months she has been complaining of it slowing down as it warms up.It was doing this at 60mph to start and now at 45mph. It runs perfect when it is cold but once it warms up it’s downhill. The mechanic is stumped. There is no code for it. The distributor seems quite new. The fuel pressure is the same when it is acting up or not. Don’t know about fuel volume. Tried replacing the map sensor and another sensor that I cannot remember, to no avail. The car has 172000 miles on it. What can be the cause?
I would be suspecting the catalytic converter. If it’s partially plugged, it will limit the top speed of the car by quite a lot. Other types of exhaust restrictions can do the same thing, but the catalytic converter is the prime suspect. There are a few easy ways to check for an exhaust restriction like this. First is a test you can do yourself. Rev the engine. If the engine cannot rev very fast but otherwise seems normal (may have a “vacuum cleaner” like sound), you probably have an exhaust restriction. If you’re still not sure, your mechanic can put a vacuum gauge on the engine (if he has one and knows how to use it as this seems to be a lost art) and interpret the results to determine if the exhaust is plugged (This test can give a lot of information about the engine to someone who knows what they are doing). Some last resort measures to diagnose this is to remove an upstream oxygen sensor to give the exhaust somewhere else to go and operate the vehicle (this is a good, free test for a DIYer with limited tools and knowledge if that’s you). If the top speed comes back, you have found your problem. Last resort is to remove the converter and inspect it for obstructions, meltdown, or broken substrate.
I have a early 90’s of the same make/model. First of all the engine is supposed to slow down as it warms up. It is part of the warm up sequence. The fuel mixture is enriched and the idle speed is faster when the engine is cold, then as it warms up it goes back to normal. But this is at idle speed. Shouldn’t be noticed in most cases at driving speed.
Still, I think the first place to look is the mixture. The fuel pressure (at the rail) isn’t supposed to remain constant as I recall. It is supposed to change with engine temperature. That is what may be the problem. I forget what it is called, but there’s a gadget part of the emissions system that does this. Suggest to find the manufacturer’s repair manual and look at the emissions system chapter. It may be as simple as replacing that gadget.
That said, it could be the O2 sensors and/or the cat. Those would be my next guesses. There’s a possibility of a head gasket problem too. Might be worth doing a leak-down test or at least a compression test. Check the oil and radiator fluids too, to see if you see any signs of cross-mixing. Cross your fingers it is not that!
Best of luck.
Your vehicle is a 96 so it has the OBDII engine management system. So if there were a problem with the catalytic converter or with any of the O2 sensors the Check Engine light would be on.
If the problem occurs after the engine has warmed up, then there may be a problem with the coolant temperature sensor for the computer. If this sensor has failed where it’s telling the computer the engine never gets up to operating temperature when it actually does, the engine will run rich and the ignition timing will be off. And this will effect engine performance.
The coolant temperature sensor is one of the primary inputs into the computer. And as long as the signal from the sensor is constant the computer accepts that signal and the Check Engine Light doesn’t come on.
Automatic transmission? Any chance that the transmission could be slipping a bit once warmed up?
Just something for consideration if the other suggestions don’t pan out.
Change the fuel filter…Disconnect the EGR valve and see if there is any change or improvement. This may throw an EGR failure code which you can reset later…