Engine miss fire @3,500 rpm ???

[scooter55]

Hello first time poster here. I have had my 1980 Fi 2000 since 1985 it has been a great car. It has 90,000 miles on it now. It has been very reliable. About five years ago I treated it to a great paint job. This year after blowing a head gasket I rebuilt the engine. It was sitting for five years after the head gasket blew. I just got the motor back together again. The motor was in pretty good shape I was able to use standard bearings and rings. Well I put the motor back in and had it running for about a week now. It seems to have an intermittent problem. Once the car warms up it occasionally starts to miss very badly at 3500 RPM. I hooked up with fuel pressure gauge to the line going to cold start valve and ran the car until it missed. The fuel pressure stayed at 30 pounds. so I was able to rule out fuel starvation. that leaves an electrical miss. I put on a new distributor cap, wires, spark plugs, rotor, distributor rebuild kit. I also switched out the fuel pump. I had an extra Coil and control module set up so I switched out out also. I also had an extra mass airflow sensor so I tried switching that out as well. I also tried loosening the gas cap to relieve pressure inside the tank thinking that it was causing fuel starvation. That's when I finally hooked up with fuel pressure gauge so I can at least rule out fuel. The timing is 10° before top dead center check with a timing light. Motor runs great when cold probably for the first 15 minutes. then it will start to miss at 3500 RPMs. When it begins to miss if you keep your foot on the gas it will consistently miss. It acts like it has a rev limiter. And then the next time you shift the miss will go away and run cleanly to 5000 RPM no problem. If you shift at 3000 RPM you would never know you had a problem. I cleaned the ground for the fuel pump that Connects in the back of the trunk by tail light. Also replaced the fuse on the brown and white wire that controls the fuel pump. The engine temperature is 190° and runs perfectly. The Car will Drive perfectly the first 15 minutes without a miss if you then shut the car off and let it sit for five minutes and then take off it will certainly have the miss @ 3500 rpm. I have run out of things to check and was hopeing the vast knowledge on this forum could help me out. I was wondering if a valve could be hanging up in the guide, causing it to miss any thoughts on that. Any guidance on this will be greatly appreciated. I haven't gotten a chance to drive the Fiat for five years so I'm really looking forward to get it back on the road reliably. When I figure out how to put photos up I will put up a picture of my baby.

[So Cal Mark]

did you have the injectors cleaned? Sitting for 5 years will leave them clogged with varnish. How is the flap in the afm? Does it work smoothly? Does the advance in the dist work smoothly?

[scooter55]

Forgot to mention I put in new injectors . The flap seems to move fine.I even switched it out with a different one. I checked the advance with a timing light and it seemed to advance the timing smoothly. Not sure when there is a load on the motor and it heats up if it is advancing. I do not have an extra distributor to switch out. I wish I did I would like to eliminate the advance being the problem. I will try switching out the ecu next but I do not think that is the problem. If the valves were to tight could that cause the miss ?

[scooter55]

I changed out the ecu and it was not the problem. The problem seems electronic ,but I just rebuilt the motor so my question is could the valves adjustment cause this problem. Or maybe a valve hanging up in a guide? My machinist set the valves when he did the valve job. I have not rechecked them yet. HELP !!!!

[RRoller123]

Bad/worn Air Flow Module? There is a post here on the forum on how to adjust the arm so it wipes in a fresh area of the substrate, that seems to address issues such as this.

[azruss]

the valve hang up thing is a real reach. hard to think the guide would be so tight as not allowing the valve to close at just one specific rpm and then allow it to close at a lower rpm. it would be more likely that it would slow down the closing creating a problem that would start earlier and get worse as the rpms went up. 2 things come to mind. first fuel. if you have a piece of junk in the fuel line, like a damaged inner wall that has created a flap so the faster fuel goes thru it, the more it blocks the fuel, finally cutting it off at 3500 rpm. if the cut off is at 3500 regardless of the gear, then this doesnt make sense either as the lower gears require less fuel. this leaves an ignition issue. take a look at your centrifugal advance and see if the weights are smooth and the springs are ok. may want to try retarding or advancing the timing while missing and see if it alters anything. if it does, then it may be an advance issue. again, this is reaching. If it is rpm specific regardless of what gear it is in or whether the engine if free reving, then you are narrowed to ignition timing. it seems vaccuum advance would be gear related.

[scooter55]

Thanks for the imput. The problem is in every gear. I tried changing the timing ,first one way and then the other. It made no difference as to when the miss happens. I just put on another new rotor and cap with no joy the bad fuel line makes sence but I hooked up a pressure gauge and ran the car and watched the gage as the car was missing and it still was 30+ psi it did not drop. It only does it when the motor is up to temp. When cold for the first 20 min it runs great. The AFM door works smoothly. Only thing I can think of left is valve adjustment out , but I do not know if that would cause a miss that seems like a rev limiter kicked in. No backfire just falls flat on its face. I will try any ideas you have I have exhausted the limits of my skills!

[scooter55]

One more question could a catalytic convertor blockage cause the problem I am having? The car sat in the garage for 5 years could that damage the catalytic convertor? I could disconnect it but I would rather not mess with the bolts that will surely brake off. But if you think it could be an issue I would take it off.what do you think?

[azruss]

a clogged cat will produce lack luster performance and a motor that doesnt want to rev. shouldnt cause a miss. valves out of adjustment wont cause a miss either. Try pulling each plug wire while missing and see if it is one cylinder that is the problem. switch plug and wire with another cylinder and see if the miss moves. if it doesnt move, the problem is not electrical.
on the fuel side, you can have a clogged hose issue with one cylinder and would still be reading good pressure. this would imply a hose issue between the injector and the fuel rail.

[scooter55]

Thanks azruss, all the hoses on the fuel rail are new but a piece of something could have wedged it self down in there. I am not sure I understand why it would work fine when the motor is cold? You are right about the pressure being good if the problem is in one cyl. I had not though of that. Also i was getting my perssure reading from the hose going to the cold start injector.which would by pass any clog in the injector.I guess the next step is to pull the fuel rail off and have a look. I am not sure it will miss when not under a load. I am reluctant to rev a fresh motor to 3500-4000 rpm. The plugs are new so pulling them to see how they are burning is most likely not going to show me much. I would really like to know which cyl is the problem . If it is one cyl. Please keep your thoughts coming at least it will give me something to try.

[majicwrench]

a better way than pulling plug wires to know which cylinder is missin..
Take 4 short 3-4" pieces of fine wire, anything will work, the smaller the better, stip 1/2" off both ends. Pull plug wire at cap, insert wire, shove plug wire back on. Do this to all 4. Now you have a cap with 4 wires sticking up. If you take a test light ( or anything clipped to ground) you can kill cyllinders by touching the exposed wire. Obviosly you can't hardly do this on the road
Puling wires at plugs is a good way to get shocked and the current has to go somewhere, if it doesn't go thru you it will arc inside the cap or jump somewhere.

BUT it is likely NOT just one cylinder. One cylinder dropping out would not make it act like a rev limiter.
Valve adj can certainly make it miss. But if it does it sometimes and not others, not likely.
Exhaust system absolutely could be causing you problems. Does the sound from the carb increase when it is acting up?? Baffles/ hunks of convertor etc can be flopping around inside exhaust causing blockage. Again,dang hard to test on road under load.

Keith

[scooter55]

Thanks Keith, that's a neat trick to test the cyl I will definitely try that. I think at this point I may as well disconnect the cat. Does anyone sell a test pipe? I may as well get rid of it at this point. Not sure what you mean is the carb louder. It's an Fi spider. Do you mean induction noise? I just did a compression test and all the cyl were dead even @ 130 .

[majicwrench]

When exhaust blocked up they tend to roar thru the intake. Again, gonna be hard to notice under load on road. Can be things other than convertor. Find problem before ripping off.

Did you test Fuel pressure under load?? You said you watched gauge as engine was missing.....will it act up sitting in bay??Testing it sitting still is NOT a good way to know if you are getting enough fuel. Get enough fuel hose so you can secure gauge to windshield wiper, go out on road and make it act up. Gauge should never go down at all. Not sure if 30lbs pressure is enough, check spec.

[timinator]

Factory temp gauges are not very accurate. The engine coolant temperature sensor is accurate. The ect can over time start to fail or develop glitches. When the ecu does not receive a signal from the ect it can limit engine performance. It is possible that when your temp gauge first indicates the engine is at 190 deg. the ect has not yet failed. It seems a little more running time increases the actual coolant temp to the point the ect quits working even though the temp gauge is still reading 190 deg. You can probe the ect to find out if this is the case.

[So Cal Mark]

I'd try wiggling the injector connectors. It's very common to find the connector worn and losing connection at the injector. The pigtails can be replaced pretty easily