Hesitation at partial throttle and cruise control inoperative
 

As I was heading to the track this past w/e cruising down the highway my car started hesitating. It felt like I was towing a trailer feeling it tug on the car, then let up. Additionally the cruise control wouldn't engage. I turned around and went to Mazcare (a respected Mazda independent) where we connected it to the Mazda diagnostic computer system. There was a "DSC Light" code and a code for the TPMS. I know the TPMS code is from running at the track with my R compounds and no TPMS sensors. I am guessing the DSC is just because I have run with DSC disabled at the track too. Anyway we cleared the codes, ran the car with data logging turned on the computer and found everything normal. We then cleaned the MAF and reset the ECU with the brake stomp procedure. I headed out and the car ran fine all the way for the 4 hour drive. Cruise control worked fine too. Sat morning heading to the track it started again. I decided to run it on the track and see how it would run., It ran fine wide open. It just didn't like partial throttle street driving. It hasn't cleared up either.

This problem occurs when the car is cold or hot. The coils, plugs, and wires are only about 7K old. This morning I pulled the coils and they look like new. Plug wires are seated properly, MAF screen in intake is intact, I reset the ECU by disconnecting the battery and doing the reset and brake stomp procedures again to no avail. The car has 65K on the odometer. I don't think I am down on power at all as I ran the best laps I have ever run at the track I was at. The orange cruise control light comes on when I turn cruise on but when I push "set" it does not engage and the green "cruise" light doesn't come on. If I push it repeatedly I can sometimes get it to engage for a split second. It then disengages and continues the surging/hesitation.

Any ideas would be greatly appreciated by those who have seen this problem or had this problem before. I have found threads by TeamRX8 and ULLOSE describing similar problems and have PM-ed them for suggestions. Of course I will update everyone on what I discover in the end.

Umm.... Is it possible you have a bad CAT?

How about a small vaccum leak? In the old days the cruise control used a vaccum.

A vaccum leak may cause the problem down low but not wide open.

Check the plug under the TB to see if it came loose or is off. I think u have an aftermarket intake.

It has been know for the plug to come off.

Razz - great suggestions. Thank you! I don't think it is the cat since it works quite well at the top of the RPM range with no apparent power loss.

As I understand it, the cruise control is not vacuum operated. I would guess that the cruise control problem is a symptom, not a cause.

The little red cap under the throttle body is on and quite snug.

Anyone else? Please...

Still having this issue. Anyone with expereince that resolved it?

TA, what is your vaccum reading?

Charlie hooked up a meter anf it read 16 hot which he said was fine.

I am pulling 18.5 hot. 16 seems marginal

it would have to be lower than that to be a major issue

have you or Charlie tried going through the Item 11 checklist in the Troubleshooting section of the factory service manual?

the cruise control switch circuit is tied in with the MAF and Baro sensors, might be something to take a harder look at

unfortunately there are many things that could go wrong without a MIL/code, but you really need a WDS to see and test all the PIDs and sensors throughly. I can see some general stuff with the extended RX-8 software for my Harrison CANScan, but there's still a lot of things missing that I wish it had :(

actually I think the WDS is being phased out for the Ford/Mazda NGS? I read a Mazda tech post somewhere that said they haven't seen a WDS update in over a year ...

Did you check the throttle body? It is electric and may be failing during part throttle, not very common but I have seen a couple fail with similar complaints.

On a side note the WDS is obsolete, very slow and only really good for compression testing on rx-8s and 7's. Mazda now uses the M-MDS for reflashing and diagnoses/datalogging.

The car is at Mazcare while I am out of town on business. Charlie has the full computer thing that a Mazda dealer has. He is getting a throttle body to try from a source he has. In his conferring with other Mazda dealer techs, it is quite possible to have an issue with the throttle body and no code is triggered. Apoparently they have seen it on a Mazda 6 in the past. So we'll try that out. I'll keep y'all posted...

Buy an iDS from a Ford dealer...they seem to be 1/2 the price that Mazda wants for them.......

still not cheap, some for sale on eBay

Worth every cent when you need it. Not quite as nice and easy to use as my old MoTeC datalogger was but not bad. Don't waste any time or money on a WDS and absolutely don't buy an MDS from Mazda. If anyone is really serious about wanting an MDS/IDS , contact me.

Ok, spill the beans on what the fix was :)

inquiring minds would like to know

It wasn't the throttle body but some switch on the tranny. I am not as versed on this but it didn't throw any codes and Charlie was able to diagnose the faulty sensor using the IDS system. Problem seems taken care of.

i have a spare one. :)

beers :beer:

it is a guess, but i am going with what ever you need to fix you problem.. :)

dont read to much into that.

beers :beer: