rdiamond2000

I have a '94 manual touring with only 30K miles and it has suddenly developed a severe high RPM stumble. Even in the driveway the engine basically will not go above 4,000 RPM. And at that point it is misfiring badly. Under 4,000 RPM it seems to run fine. There seem to be so many potential causes from bad grounds, to a secondary turbo problem, TPS, fuel filter, plugs, bad ignitor, Vacuem leaks, I really don't know how to intelligently find the problem and fix it. I have the shop manuals and they are not very helpful in this because there are so many potential issues. I am moderately skilled with a wrench but not that experienced and live in Puerto Rico where I am having trouble finding a mechanic for this car. I don't mind trial and error and would very much appreciate any helpful suggestions on where to start. I have ordered new fuel filter, plugs and wires and will replace those. I am trying to figure out where the grounds are and looking for suggestions from some of the experts here on this problem. Thanks for any help!

Sgtblue

iTrader: (16)

B

Cgotto6

iTrader: (8)

Plugs and plug wires is a good place to start as you have stated is in process. Seems ignition related if it flat out wont rev above 4k.

Retserof

iTrader: (1)

How long has the problem been present? My first suspicion would be bad gas due to water in the gas either from condensation in the gas tank or from the gas station. Have you added new gas from a different station since the problem began? Also, check that the plugs are in tight and the wires are on correctly.

rdiamond2000

Thanks for the suggestions. Much appreciated. I will change out plugs and wires and the fuel and air filter (those last two are maintenance items although a partly clogged filter could (I think) reduce gas flow. Then maybe stay on the ignition side. Could an igniter cause the issue I am describing?

rdiamond2000

Thanks for reply. The gas is about six weeks old and the problem started three weeks ago. I did fill up at a new station with this last tank, and I am down to a 1/4 tank. It would be easy to run out most of that gas, add some dry gas to deal with condensation that may be in the tank perhaps and fill up the tank. For plugs and wires I will put the new ones on. Fingers crossed it’s as easy as those items! Thanks again for the suggestions.

rdiamond2000

you are a man of few words….the solitary “B” is a mystery suggestion! Not sure if you were trying and somehow the comment did not post

apexhittinbull

iTrader: (21)

Less likely of a fuel problem but more of a ignition problem.
Alt, ignitor, plugs, and or wires.

jza80

iTrader: (32)

Not sure what engine management you have, but the stock ECU in limp mode could get you the issue that you are describing. If you are still using the original ECU, you might want to try checking to see if you have any fault codes.

GP6

Plus one for checking the fault codes. Somewhere on this forum there is a chart of which fault codes can cause limp mode, although limp mode is more like a fuel cut and not a random misfire. CAS connection can cause high speed stumble also.

rdiamond2000

OK so I managed so far to change out all spark plugs, three out of four ignition wires (I could not get to one of the wires and It appears that I need to remove the intake manifold to do that. Have not had time to do that yet) and I also pulled off the grounding wire I found that attaches to the rear firewall and comes off the intake manifold. I polished that up with a wire brush. So far no change in the car. Still stumbles. I am wondering if there is another ground somewhere else I should check and clean up but I don't see any.

I am also wondering what happens in the engine at 4,500 to 5,000 RPM? Does some secondary system come on line or does the ignition timing change? That would seem like a likely suspect but I am simply not sure what might change. Especially in the ignition. I am also wondering if secondary injectors come online at that point in time and if that could be it.

Overall it seems like there are a lot of parts it "could be" and I could spend a lot of time and money replacing parts that might be working perfectly well. Hopefully some smart people here can help out with likely suspects.

rdiamond2000

BR7EQP Leading Plug

BR9EQP Trailing plug

BR&EQP other leading plug

BR9EQP other trailing plug
Here are photos of the old plugs in case anyone has any ideas about what these may say about the engine.

So far I have changed out all spark plugs for onew NGK br7 and BR9 plugs, changed three out of four ignition wires with new NGK RC-ZE30 wires (I could not get to one of the wires and It appears that I need to remove the intake manifold to do that. Have not had time to do that yet) and I also pulled off the grounding wire I found that attaches to the rear firewall and comes off the intake manifold. I polished that up with a wire brush. So far no change in the car. Still stumbles. I am wondering if there is another ground somewhere else I should check and clean up but I don't see any.

I am also wondering what happens in the engine at 4,500 to 5,000 RPM? Does some secondary system come on line or does the ignition timing change? That would seem like a likely suspect but I am simply not sure what might change. Especially in the ignition. I am also wondering if secondary injectors come online at that point in time and if that could be it.

Overall it seems like there are a lot of parts it "could be" and I could spend a lot of time and money replacing parts that might be working perfectly well. Hopefully some smart people here can help out with likely suspects.

rdiamond2000

Here are photos of the spark plugs I pulled in case someone can read these and give any input on what info may be in their appearance. And I replaced the plugs with new NGK BR9 and BR7 plugs and new NGK RC-ZE30 ignition wires.

GP6

I would check for codes, and refer to the Factory service manual to find out what the codes mean. There is a diagnostic box close to the battery to get to the codes. I found everything by searching this forum when I was having issues.

rdiamond2000

So I found a very experienced RX7 mechanic here in Puerto Rico where I live. The issue was the throttle position sensor. Once that was replaced the engine no longer stumbled and the car runs perfectly once again. I also changed a few parts on my own but those did not solve the problem - namely I replaced the spark plug wires, spark plugs, and wire brushed the grounding strap from the engine to the car body but those changes did nothing to my high RPM stumble issue. Thanks for all the comments and hopefully this will help someone else. And in case someone needs an RX7 rotary engine expert Berto of Vivo mechaniica in San Juan was great. Phone (787) 385-8780. He also orders engine seals in bulk (10,000 at a time) and offers them in sets for resale. He also builds high performance engines although I am keeping mine stock.