98 Maxima runs great, then not so good.

[LylePfeifer]

My son's 98 Max runs great. He made 3 stops in an hour and a half yesterday shopping. Last stop, he took off and had to floor it to get up to 40 MPH. Erratic low idle,bogging performance. Went 5 blocks to his apartment and parked it. Went out a couple hours later and it ran fine again. I had a knock sensor code, replaced the sensor. Had multiple miss-fires and changed from 20000 mile Autolite to NGK Iridium plugs. New rear Denso coil packs, fronts on order. Pulled the intake plenum to make sure all carbon was cleared out as I got a EGR code, 20000 miles ago the EGR tube was blocked. Pulled off several EGR parts and tested them, all OK. New PCV valve and tested for vacuum leaks. All new fuel injector seals and replace those little plastic things on the end. Name escapes me right now...pintels? Just put on new pre cats and new cat. New O2 sensors. 211,000 miles on car. I did find a little oil in the center cylinder spark plug valve cover hole, the rubber donut was out of alignment and I fixed that. It never seems to do this after it is started, only after it has run for a bit. Of course it never will do it for me and sometimes will go for a week or 2 and it only happens once or twice and then runs fine again. Cleaned the MAF sensor with MAF cleaner. Cleaned the throttlebody with throttle body cleaner. Thing runs great, except for this and a small oil leak from the rear right side of the engine that is always annoying. Any ideas out there, as I really don't want to keep throwing money at this thing. NO CODES OR CHECK ENGINE LIGHT when this happens. I am thinking it is something that is not monitoured by the ECM. Fuel filter is not that old and if it plugged I wouldn't think it would unplug and plug again. Fuel pump in original. I did put in a new ignition switch also.
Thanks in andvance for ideas.

[NutriaforBreakfast]

I would use a multimeter to check the throttle position sensor and some electronic cleaner for the sensor and connectors

[zul8tr]

I hope you bought this car cheap with all the parts you put into it? With over 200K miles there could be many things that cause this behavior, like corrosion in wires, connectors etc. Example: my 1992 300zx NA had to have the injectors replaced to the pintleless design post 93, they failed with E fuel and corrosion. When I got in there to install new required connectors for the different injectors I found corroded green copper wires to the connectors, a contributor to failure IMO. I had to cut back to fresh wire for good connection.

Not sure this will fix yiur issues but it might help get cleaner signals to the electric parts, I did this to my Z and my I30 as PM. Start pulling all the connectors you can get at ONE AT A TIME. Clean them and put a nice gob of dielectric grease in the female part and reconnect. Mark the connector with a sharpie pen so you know it is serviced. Especially the injector connectors and coolant temp sensor they get corrosion. Also any grounds and the battery terminals - pull them clean the batt posts and the female part of the + and - cables, put dielectric grease on the post and cables and reinstall - this prevent electric arcing corrosion.

Since you have a warmed up engine issue have you checked the coolant temp sensor ohms? Have you checked the thermostat for correct temp control? A thermostat modulates during operation based on load to maintain a temp range and if the coolant isn't within that range the ECU will send different voltage signal to injectors and other components which could result in your problems? The easiest way to check thermo is to shoot it with an infra red temp gun when fully warmed up. The coolant temp spec is in the Service Manual. If you replace the thermo do a stove top test (see Manual) and heat the water and note temp when valve just opens and full open temp and compare to specs. New has been known to be out of calibration. I would only use OEM on this part.

As suggested properly adjust the TPS.

Service Manual 1998 Maxima:

http://www.nicoclub.com/FSM/Maxima/1998/

Good luck

[LylePfeifer]

OK, I gave up and went to see my go to guy when I can't figure things out on my own. George, my guy, said you know it’s been hot and I have been changing fuel pumps for over 2 weeks. In 100 degree temps if you don't keep that tank at least half full, you’re going to be using hot fuel to cool that fuel pump. But I told him I just drove the car 2 hours without a hick up. Let it set 15 minutes and it stored a multiple miss-fire code. He said it did that because the fuel pump is failing, seen a bunch in the last 2 weeks. Get a pressure gauge and drive the car, watch it and see what it does. I went to Oreilly's and rented one. Hooked it up and low and behold, it was supposed to be 35 PSI. 28 PSI was the best it got and if you floored it, down to 15 PSI. If you pull the Vacuum off of the fuel pressure regulator, it should jump up to 43 PSI. Nothing, stayed at 28 PSI. I ordered a new fuel pump, a new fuel pressure regulator just in case and was staring at the cam shaft sensor as it was misting a little oil from the tired o-ring seal. I pulled it out to change the o-ring, cleaned it up and notice a crack from one of the molding holes half way across and right through the metal tab. So I put that on new too. I checked the new fuel pump, 35 PSI and flip the throttle, no change. Rock solid at 35 PSI. I pulled the fuel pressure regulator vacuum line and right up to the max of 45 PSI. Car has been running fine. Now I have to go find George and had him a $50 for 10 minutes of his time. Anyone with car stalling problems, you might want to try the fuel pressure gauge and see where you’re at. George said he has seen as little as 2 PSI low cause all kinds of problems.......I never thought a car that would run for 2 hours solid would have had a bad fuel pump.