Stumble at high rpm
 

Last nite I went for a cruise with some other 7's and we did a couple hi-speed (140+) runs.
Today my car has a slight stumble in 2nd and 3rd gear (haven't gone higher yet) in the upper rpm band. It's very mild around 4500 or so and gets a little more noticeable as the tach climbs. It's like subtle misses.

My plugs are not that old and I'm about due to change the fuel filter.

I have Autometer gauges - the A/F ratio is towards the rich side mostly yellow, bordering on green and I'm boosting about 8-9psi in 2nd and 9-9.5psi in 3rd.

I'm going to change fuel filter and plugs but was wondering what else it could be if this doesn't fix it?

plug wires and plugs would be the first thing I would be looking at.

Then engine grounds

Something got knocked loose during those 140+ runs, I'd do a nice through inspection of vac. lines and wires. Have you rewired your walbro? Definatly replace the f. filter, just takin stabs in the dark:o: .

I did another test tonite and this is what I noticed:

While driving on the freeway if I slow to about 65-70, drop to 3rd and floor it, I get the stumble slightly (starts at around 4500rpm) and it gets a bit worse as the rpm climbs to 7000. It's like it's missing just slightly - nothing like a fuel cut.

When I get off the freeway and run thru 1st, 2nd and 3rd it only does it occassionally. On most attempts it pulls thru these gears without incident.

So I went back out on the freeway and same thing, if I slow down, drop to 3rd and floor it it's more pronounced.

I always fill with 93 octane, I've had Magnecor wires (which I checked for fit tonite) for 3 1/2 years and I added a fuel injector cleaner 2 weeks ago.

I'm suspecting fuel filter (I don't see how that is very likely unless these things get clogged at 20-25K miles) or fuel injectors. I'm not running lean and boost is 9-10psi in 3rd.

A couple of months ago I used to get the hot start problem - which may have been injectors. How do I know if it's injectors after I replace plugs, wires and fuel filter?

Any other ideas?

I would say reground your engine block. The FC's has pathetic engine and ecu grounding. My hesitation problem was fixed by adding grounds.

I suspect your hesitation is when you are under full boost, correct? If the engine isnt grounded well enough, there wont be enough voltage going through the spark plugs(wont spark without a good ground). That, and the boost is preventing the mixture from being properly ignited. Its definately worth a try, only a few $$$, and never hurts.

I'll give that a whirl, what's the proper way to do this???

I had a bunch of 4 guage wire laying in my garage from an old system of mine. I just ran that from the - terminal on the battery to the bolt that holds the front engine lift eye on. I put the ring terminal between the engine block and the lift eye, and tightened it down. Worked great. For best results, you should also add a new chassis ground, battery to a good chassis ground. You have to scrape the paint off to get a good chassis ground.

And if youre feeling especially productive, you might also want to reground the ECU (4 black wires in the middle plug, if I remember correctly) and the boost sensor(brown/black stripe).

i have the same problem my car stumbles hard after 4500 rpms. i noticed it happens more in the mornings or when its cold, then tapers off after it heats up.

is this the same with you? or is all the time?

Check the TPS, especially WOT resistance.&nbsp Check to make sure it's NOT over 6.5k-ohms.



-Ted

well i went and did this, grounded battery to engine block, then battery to frame, still having problems, i know this may not be the probelm but explain about grounding the ECU more i'd like to try that i looked for the black wires but only found a couple not 4, mines a 87 turbo

My S5 had 4 black wires out of the ECU. S4's might be different. Also, did you check the TPS? It being slightly out of adjustment it will cause those problems.

Thanks for the help guys.

I ordered some NGK plugs of a different sort today (B8EGV -leading & B9EGV - trailing) and will pop those in Tuesday and replace fuel filter.

I dumped in a bottle of Chevron FI cleaner today and did some blasts in 3rd on the fwy a few times. I didn't notice any hesitation, possibly because it's warmer today and I'm not boosting high enough to duplicate the hesitation, so I'll try tonite as well.

If, after plugs and f. filter it's still stumbling, I'll try the additional ground. If that doesn't do it - off the injectors go to RC Eng.

Ted whats the TPS? I have an 86 N/A
I am having same problems... just a slight miss - I havent checked plugs or anything else yet but when I do get some time to mess with it I want to have my options lined up.

We ran into a problem with a friend's 1987 Turbo II which we could NOT figure out a high RPM hesitation 6,000RPM+.&nbsp We went over the entire ignition system and found nothing.&nbsp The problem was eventually tracked down to a TPS that was out-of-range at WOT - it was reading 7.0k-ohms at the time.&nbsp Brand new TPS usually run about 4.5k-ohms.&nbsp When the TPS wears out, the resistance at WOT start to creep upwards.&nbsp In my experience (I've run into a few more problems that were the same cause), 6k-ohm seems to be the limit.&nbsp You can push it to 6.5k-ohms, but this is iffy.&nbsp Anything over 6.5k-ohms at WOT will definitely throw the ECU into fits - I don't know why, but I've seen it and fixed many of these problems with a good TPS.

Now, before you go blaming the TPS, make sure the ignition system is in top shape and your fuel filter is relatively new.&nbsp Worn spark plugs, work spark plugs wires, arcing wires will all cause high RPM hesitations.&nbsp A dirty fuel filter will cause loss of top-end power, and those with experience can tell the difference between a fuel mis-fire and an ignition mis-fire - most people can't tell the difference.&nbsp The TPS is the LAST thing that should be blamed - it's a >$100 price tag to replace with a new unit from Mazda!



-Ted

I ran my evening test tonite (it was about 55 degrees out). got out on the freeway and I would slow to about 65, drop into 3rd and floor it - same results - still stumbling and I have almost used all the gas that had the Chevron FI cleaner.

Since it was cool out I saw upwards of 11psi at 5500-6000rpm in 3rd, mostly around 10psi though.

This stumbling/hesitation problem definetely is more noticeable as boost increases....does that point to anything specific?

When I was headed home on a 45mph 2 -laner I can drop to 2nd and full throttle thru 2nd and 3rd with no stumble or very slight in 3rd.
It only does it on the freeway...

Are you sure your not getting detonation? Try backing the timing off a couple degrees, or running slightly rich to see if it goes away.

As I mentioned further above my Autometer A/F ratio gauge is indicating in the high section of yellow and sometimes bordering on green (yellow is good, green is rich).

I'm popping the hood in a few mins. this morning to check for any loose hoses/connections.

That brings up another question - I am overdue to replace my O2 sensor, could this have anything to do with the stumbling/hesitation?

bump... Fedrex, just wondering how you solved this problem. I too have the same thing.

where did all the smart people on this forum go?

not sure if it's your problem, but i just replaced my o2 sensor and it made a huge difference in the stumbling issues i was having. Mine is n/a but I'm sure the same applies to a t2.
Patrick

the o2 sensor reading is not used by the ECU when the car is in open loop.