Hesitation at middle RPM under moderate load
in [Technical]
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From: Hachiroku ハチロク on 22 Jul 2010 23:16 On Thu, 22 Jul 2010 18:52:24 -0700, nm5k wrote: > Course, it could be running lean due to other problems, but > keep the possible carbon in mind. It's pretty common. > What happens is the carbon snow cones behind the intake > valves, and when under a heavy load, or when hot. OK. I meant to mention a couple things. It has been REALLY HOT here lately, although it was hot last week and it didn't do it (or perhaps I didn't notice...) Also, since it has been hot, when I park the car I leave the ignition on so the fan can run and keep temps out of HG blowing territory. Sometimes for 20-30 minutes. I know in the old days this could cook your points, but I am not sure of the effect on modern (er, MORE modern, anyway...) cars and was questioning if this is a Good Thing... And, tonight it was under 70 degrees, and I couldn't get it to sputter...
From: Ray O on 23 Jul 2010 00:24 <nm5k(a)wt.net> wrote in message news:5e1fbbaf-4ccb-40ee-9d63-d784f7b56f22(a)c10g2000yqi.googlegroups.com... > On Jul 22, 7:14 pm, Hachiroku $B%O%A%m%/(B <Tru...(a)e86.GTS> wrote: >> The car in this case is a 1989 Mazda 626. It was running great until the >> day before yesterday, and turned 190,000 miles last week. >> >> AT highway speeds it hesitates only a little bit, unless you start going >> up a hill, then it starts sputtering. Normal around town driving, not so >> much, unless you're going up a hill in third gear. >> >> Usually occurs around 3,000 to 3,500 RPM. Once you get about 3,500 it >> smooths out unless going up a hill. > > > What you describe is what happens when the intake valves > get carboned up. The Techron was a good idea. It will clear > that stuff up, but it takes a while. > Course, it could be running lean due to other problems, but > keep the possible carbon in mind. It's pretty common. > What happens is the carbon snow cones behind the intake > valves, and when under a heavy load, or when hot, the carbon > will expand and keep the valves from fully closing. So you start > losing compression. Remove the heavy load, and it starts > running right again. The carbon also soaks fuel and can > cause lean running. > The only real way to tell if it's a fuel pressure problem is to > actually hook a fuel pressure gauge to the engine. > A car with a super carbonized engine will quit running once it > gets hot enough. We had a Camry that had that problem. > The dealer had it for two weeks and never could find the problem. > They blamed it on the wiring harness, and charged us a load > of money for not fixing the car. > I ended up finding the problem myself, which was severe carbon. > Anyway, like I say, it may be something altogether different. > But keep the carbon in mind, cuz I've noticed most mechanics > won't think of it. The Camry was in three different shops, > the third being the dealer for two weeks. None of them could > figure out the problem. > It took me a bit of pondering before I found it. The first thing > I did when I drug it back from the dealer was to check the > wiring harness, which the dealer claimed was uncheckable. :( > It took me about an hour to check every single wire in the harness. > So much for uncheckable.. Never tell an extra class ham > a wire is uncheckable..They will never believe it. I know I > didn't. :/ > I had to build a pressure gauge, and after checking the whole > car out, could find no obvious problems. But it still wouldn't > stay running once it warmed up. But I finally found the problem > after noting the loss of compression when it did it. The starter > would spin faster than usual, and often erratic due to the loss > of compression in some cyl's, but not all. I ran a long hose for > the fuel pressure gauge and was watching it as I drove. > It was fine and never dropped from normal. > If it's carbon, it will keep getting worse until the point on steep > grades it will totally quit running after a while. > Super bad, and it will quit running when it's hot, no matter > what load you are putting on it. > Keep running the Techron for a while. Two bottles worth > will knock it out enough to tell if it's carbon or not. > Maybe even one bottle if it's not too bad. > Then again, I guess it could be an ignition problem.. :/ > It's all a guessing game when you are on the interweb and > can't see the malfunctioning machine. > I doubt if the problem is related to fuel delivery if the engine runs ok in lower gears and you can get it to rev. I second the carbon buildup on the valves. Hook up a vacuum gauge, and when the problem is occurring, see if vacuum drops. If the Techron doesn't work, find a shop with Motor-Vac or that has a way to do the walnut shell blast. As Tegger mentioned, the EGR is also a possibility, but not likely to cause the seemingly random other than heat symptoms you're describing. -- Ray O (correct punctuation to reply)
From: JoeSpareBedroom on 23 Jul 2010 02:15 "Hachiroku ????" <Trueno(a)e86.GTS> wrote in message news:4c49090b$0$74748$afc38c87(a)read01.usenet4all.se... > On Thu, 22 Jul 2010 22:08:48 -0400, JoeSpareBedroom wrote: > >> "Hachiroku ????" <Trueno(a)e86.GTS> wrote in message >> news:4c48df07$0$74748$afc38c87(a)read01.usenet4all.se... >>> The car in this case is a 1989 Mazda 626. It was running great until the >>> day before yesterday, and turned 190,000 miles last week. >>> >>> AT highway speeds it hesitates only a little bit, unless you start going >>> up a hill, then it starts sputtering. Normal around town driving, not so >>> much, unless you're going up a hill in third gear. >> >> >> It means something's not right. > > Gee! Uh, thanks, Joe! Go ahead and try to prove that I'm wrong.
From: Scott Dorsey on 23 Jul 2010 09:05 Ray O <rokigawa(a)NOSPAMtristarassociates.com> wrote: > >I second the carbon buildup on the valves. Hook up a vacuum gauge, and when >the problem is occurring, see if vacuum drops. If the Techron doesn't work, >find a shop with Motor-Vac or that has a way to do the walnut shell blast. The original poster mentioned using high-test gasoline in it just because it was cheap right now. If you run enough high-test in an engine designed with fairly low compression, you're apt to wind up with carbon deposits as a result. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis."
From: larry moe 'n curly on 23 Jul 2010 15:14
Hachiroku $B%O%A%m%/(B wrote: > The car in this case is a 1989 Mazda 626. It was running great until the > day before yesterday, and turned 190,000 miles last week. > > AT highway speeds it hesitates only a little bit, unless you start going > up a hill, then it starts sputtering. Normal around town driving, not so > much, unless you're going up a hill in third gear. > > Usually occurs around 3,000 to 3,500 RPM. Once you get about 3,500 it > smooths out unless going up a hill. > > I went for the "easy fix" first and replaced the fuel filter. WHen I got > the 'old one' out (it's only 32,000 miles old...) the fuel that was in it > ran right out and was mostly clear, with a reddish tint as it ran down. > Not muddy like the last time this happened to me. > > At 152,000 I replaced the plugs, wires and all filters. > About a year ago I added a cap and rotor from a dealer's "New old stock". > > Some guesses: pressure regulator. I hope not. AutoZone price $123. > > Wires. I used genuine Autozone "DuraCrap" wires, so this was the second > thing that popped into my head. > > Fuel filler cap? Recently I noticed it spins on easier than it used to. > > Fuel...the last two fillups were at the same Gulf station, and at 93 > Octane, since it's only 4 cents more than mid grade. Around here, any gas from a Gulf station would be at least 25 years old, and 93 octane would be leaded. Could that be your problem? ;) In my extremely limited experience, I've seen only three cases where a fuel-injected vehicle hesitated: bad fuel pump, bad insulation in an ignition wire, and throttle position sensor developed a worn spot, and only the latter gave no symptoms at lower speeds. The fuel pump caused hesitation from idle, and the ignition wire caused occasional rough idle (arcing through, but the insulation showed no cracks or pinholes, even under a magnifying glass). A regular ohm or volt test didn't show anything wrong with the TPS, but when it was hooked up as a volume control, the sound was scratchy when the TPS was rotated around a certain area. Some people test by feeding it a signal through a resistor and looking at the waveform on an oscilloscope, and a worn resistor or wiper shows up as flashes of noise. I read that cars with computer-controlled ignition may ping or hesitate at only certain speeds because the timing and dwell aren't adjusted smoothly with RPM and vacuum over their full ranges, as they are with older cars, but instead the computer may adjust them a lot over narrow ranges. IOW vacuum leaks can make the MAP sensor send the wrong information and prevent the computer from adjusting the ignition properly. |