Rough idle on warm start up (20 minutes), 87 7MGTE

[Bru]

The AFM checks out ok (I have 2). The O2 sensor has 50K on it and has tested good in the past. Tonight it did the same thing so I immediately shut it down and pulled the vacuum lines to the fuel pressure regulator and the EGR with out plugging. It started normally. I will have to repeat that to confirm and begin narrowing it down.

[Bru]

I repeated my test by thoroughly warming up the engine and letting it set shut down for 20 minutes with the hood closed. Then I disconnected the fuel pressure regulator vacuum line and plugged it up. The car started and idled normally for 3 seconds then I shut it off. I immediately reattached the vacuum line and started up again and ran rough as it usually does under these conditions. It occurred to me that I had performed the same function as the High Temp Fuel Pressure Up VSV located under the intake manifold at the front of the engine. I've known for quite some time that mine was bad and did not function which means it allowed vacuum to the regulator under all conditions. They go bad by being electrically open. (no resistance at the terminals). Until now this hasn't mattered but I guess it needs it to prevent vapor lock or 10 pounds more fuel pressure making it 40 PSI for the first minute of hot startup. Here's a very informative web page about fuel systems check out page 16. It details the Toyota high temperature (pressure up) fuel control.


www.autoshop101.com/forms/h42.pdf

[Bru]

This engine heat soak issue continues to bug me. I did some research on the web and found out that heat soak maximum temperature occurs between 30 and 40 minutes after shutdown. Another Web site about boat engines recommended running the engine for a few minutes before shutting down to minimize the problem. Opening up the hood would also help but that's not always possible in public parking lot. Winter formulated gas between September and April has a lower boiling point than summer gas along with containing 10 percent alcohol, which exacerbates the problem year-round. Living in Central Florida doesn't help matters either. We all know how hot the Supra engine runs. I'm going to try a trick next time and jumper the B+ and Fp on the diagnostic block and let the fuel pump run for a minute to flush any vapor out of the rail before I try to start the engine. There is one station in town that sells old-fashioned pure gasoline at a premium price. I may try a few gallons of that if I can't find out what's going on. If that doesn’t work, I may have to swap out the igniter and /or the cam position sensor to rule those out.

[Bru]

The issues I was having with rough idling on starting a hot engine after sitting 20 or 30 minutes have basically gone away. I was thinking of replacing the cam position sensor when I thought maybe the problem is in the harness side. I noticed that the wires that went to the cam position sensor were looped down under some of the others in the harness where they branch apart as they come out of the plastic tunnel heat shield. Now that wire comes straight out on top and over to the cam positioned sensor plug. I no longer have to wiggle any wires or push on the rubber gromet that goes into the cam position sensor. I also had the P and Q letters connected backwards on the EGR vacuum modulator because I took off the cap to look inside at the filter and put it on backwards. The decal on top misled me. Those letters are also embossed on the base which are always correct. I've had some good experience with products made by Lubro Molly such as: Jectron Fuel Injection Cleaner ($5 at NAPA),Ventil Sauber Valve Cleaner ($4 at NAPA). The MoS2 Anti-Friction Engine Treatment makes the oil look dirty but it does run slightly cooler and makes the engine rev freer. I put half a can in the engine and the other half in the manual transmission where I am using 10W-30 motor oil anyway and a half a can of Lubro Moly "oil saver" to stop seal leaks. Shifts are improved. They make a concentrate specifically for gearboxes and rear ends (not for LSD though).
I got a tank of bad premium? gas and I improved it significantly by using a bottle of Lucas octane booster ($9 at Advance Auto on sale). They claim 3 to 4 octane numbers or 30 to 40 points on a tankful. Some boostaholics mix it with good premium gas for an added cushion to forestall knocking that retards the spark timing.