View Full Version : 97 Cobra hesitation under hard acceleration

12-05-2006, 07:11 PM
I have a 1997 Cobra that has developed a huge problem.

Under hard acceleration or heavy load on the engine ( especially in gears 1 thru 3 ) the engine "hesitates" as if the rev limiter has kicked in even though the rpm's are no higher than 4200rpm. It also happens in gears 4 and 5 but to a lesser degree. The problem does not exist if I apply gradual power, in fact it is very smooth w/ plenty of pull. From a sound standpoint it is as if someone stuck a potato in the tailpipe when the problem surfaces as the exhaust note becomes muffled, the car "porpoises" forward and back while it struggles to fight through whatever is holding it up. The hesitation does disappear after 4-5 seconds of struggle and a smooth powerful engine and exhaust note return. The car does have a custom SCT flash tune and does not have this problem on the dyno at all. I am searching for answers......any ideas?

The engine mods on my car are a custom air-intake, FRPP headers, a BBK x, and flowmaster exhaust. It has a MSD DIS4 system, FRPP plug wires, and an Optima red top battery. The clutch is still stock ( only 28k miles on the car ) and I have 4:10 gears. There are a host of suspension upgrades but they should not have any potential effect on the problem I have.

Thanks in advance for your thoughts and ideas. I need some help and do not want to break the bank w/ a performance shop as they spend expensive hours diagnosing the problem.

Merry Christmas and Happy Holidays to all my fellow SCMC Members!

12-05-2006, 08:23 PM
Mike my first thought would be something in the ignition, either the DIS4 or the coils.


12-05-2006, 09:16 PM
IMRC plates are sticking...???

12-06-2006, 07:20 AM
Replace the fuel filter. Make sure the MAF sensor is clean. Does the car idle fine?

12-06-2006, 10:10 AM
Check the computer for error codes. The computer could be going south.

12-06-2006, 03:45 PM
The engine idles fine. I will check the fuel filter. MAF could cause that?

I dont want to hear it is the computer....that is way expensive I would bet. Should the computer go with just 28k miles on the car even if it is almost 10 years old?

I will check the MSD system also.

Having a brain cramp - IMRC Plates?

12-06-2006, 09:18 PM
These plates open and close at or around the rpm you described. I'm sure our moderator can explain it better but basically they are there to help you with fuel mileage while you are tooling around town until you floor it, then they open up so you can take advantage of full air flow to the engine. They have been known in time, to stick, and cause the problem you are describing. I'm betting on this is the problem with your snake! :D

Red Wedge
12-07-2006, 09:33 AM
Sounds like your IMRC plates. How often do you drive the car? They could have gotten stuck in the closed position. The way they work is once the motor gets to 3250 RPMs the computer signals a box that is bolted to the underside of your lower intake to open the IMRC "flaps". It is pretty much a box with gears in it that has two cables attached to it that opens the flaps. You could possibly have both of them not opening, 1 opening and 1 not opening, OR maybe something is wrong with the cables and it is only opening partially. This in turn causes your air to fuel mixture to be WAY OFF, your computer sensing the incorrect mixture tries to correct the problem. There is a way to watch the cables while it is parked and pushing the idle past 3250 rpms, BUT I don't like doing that to a motor when it is not under load. I cleaned the IMRC's on my 1996 cobra after I first got it. It is really not a hard process but it is time consuming. With only 68k miles on it, we used over 6 cans of carb cleaner getting them back the way they should be. I could tell a difference in mine when I cleaned them. Might give seafoam a try, would be worth the money to see if that would free them up. The reason Ford supposedly put that design in was to help torque numbers in the lower rpm range, but keep the good performance these modular motors have up top when the IMRC plates open. It is probably the most complicated design of the 4V modulars they have made so far IMO. Hope this helps a little.....

12-07-2006, 11:45 AM
Great stuff guys.....I truly appreciate the time, knowledge and advice you have given me w/ regard to my problem. I will take a look at all the suggested issues beginning w/ the IMRC plates as they make the most sense to me to start w/.

I will let you know what the problem ends up being. It may take a little while as about 10" of new snow is falling today and my Cobra must stay inside intil the roads are clear!

Thanks again to all of you. SCMC members are just excellent people!

Merry Christmas!

12-07-2006, 09:05 PM
Well after reading this post, I did some searching on some of the forums about IMRC issues. It seems that there are two approches to solving the problem. The first is much more time consuming. Pulling off the entire intake, and cleaning out valves from the bottom. The second, which after reading about, I am seriously considering doing. It involves running a bottle of Seafoam through the PVC intake and smoking out all the crap in the engine. People over on svtperformance live by it. Just something to think about.

12-08-2006, 06:48 AM
I would agree that the probable cause is the IMRC's . Seafoam will be the easy method without causing any breaches on the seals of the intake plenum. I would probably do the seafoam treatment first before removing the intake plenum. If that does not cure the problem, there could be something wrong with the gear drive in the IMRC controller.

It would not hurt to complete a check list of items after the seafoam cleaning process. Inspect the throttle plates and MFA sensor, clean them if necessary as well as inspect your plugs for wear. I would also consider replacing the fuel filter if it has not been done within 16k to 20kmiles.
After cleaning with seafoam, the engine may become hesitant in response but that should not last too long. It will take a few minutes after the cool down period to regain a stable idle since the seafoam is a type of fuel. The engine will want to stall at start up after the cleaning process and there may be some heavy smoke from the exhaust but from what others have said it should clear up.

Also, after using the seafoam, I would change the engine oil as precautionary measure since the seafoam is a fuel based solvent, any circulation of fumes from the solvent may find its way into the block through the PCV system. Wait until after the engine is running normal again before you change the oil (after the first road test).

12-14-2006, 07:32 AM
Okay....I pick up the car this evening.

Turns out that a dirty MAF sensor, a new set of spark plugs for good measure, and a new and improved fuel pump have solved the issue. The IMRC plates and MSD are fine and functioning as they should. Jeff & Tammy Chambers at CRT Performance in Norwalk (www.crt-performance.com) ran through every system on the car and they all check out. I left it with them a couple extra days ( I trust them w/out condition )so they could put some test miles on it and they confirm the issue is gone.

Next up, Jeff is putting together a design for a custom rollbar that will enable me to keep the CDC light bar on my 'vert. Hopefully we can install in early March before 411.

Thank you all for the insight and advice. I appreciate it and wish you all Happy Holidays and Merry Christmas!

12-14-2006, 12:19 PM
That is great to hear. I think my misfire in cylinder 5 was a one time fluke when the previous owner did the compression check, as it has not popped up again. I think I also got my alignment issue straight, just need some time to fix. Now I just need winter to pass quick.:D

12-15-2006, 06:06 AM
Glad to hear it is running the way it should.

Fuel pump, that was a good call :thumbsup: If the same or similar issue pops up again, you should have the fuel sending unit tested or replaced. The fuel sending unit is what controls the fuel pump and regulates the duty cycle of the pump in relation to signals from the ECU. If it overheats it may momentarily shut down or not respond as it should. I believe it is also the same part that has the inertia switch on it which is supposed to open in case of an impact to turn off the fuel pump. If the switch is faulty, that may cause momentary loss of fuel pump activity from the change in inertia when under acceleration. I am sure that would also cause other issues than what you have experienced.