Engine runs, terrible stumbling / hesitation

Keep it on topic, it will make it easier to find what you need.
CarlosJavier

Re: Replaced injectors, now it won't start

Post by CarlosJavier »

First drive - Thanks for all your help!!

I cleaned up the two FI grounds on the intake plenum and a ground wire that goes from the motor to the shell with some sandpaper. Then I connected some jumper cables from another car to the Spider and was able to start it up. I got it to idle smoothly and rev throughout the whole RPM range (no load).

When the engine was warm I turned it off and cranked it again - started right up. Repeated this several more times and no hiccups. We'll see how it does when it's cold.

Then I drove the Fiat for the first time; it was pretty sweet. However, for some reason it falls on its face when it hits 2500-3000 RPMs under load, so I gotta look into that. Any suggestions would be greatly appreciated. Thanks again!
CarlosJavier

Re: Replaced injectors, now it won't start

Post by CarlosJavier »

Just checked the distributor - the pickup wires are very corroded and falling apart; this might explain the sudden loss of power/hesitation...
CarlosJavier

Re: Replaced injectors, now it won't start

Post by CarlosJavier »

Replaced the distributor pickup; have strong, consistent spark at the coil. Set ignition timing to ~10 BTDC. Drove the car - problem persists.

The issue seems to get worse as the car warms up. At first, with no load, the engine will rev all throughout the RPM range. After a little bit, it will rev but won't hold the engine speed if I keep a stationary throttle input.

When I drive it, at first it will go up to a max of about 3000 RPMs, then hesitate and drop engine speed. When I get on it again it'll hit a max of 2-2.5k RPMs, then hesitate and drop engine speed again. This will happen several times, dropping more and more engine speed each time until the car is barely driveable. The more load on the engine, the stronger the hesitation and bucking gets.

The problem gets progressively worse, eventually preventing the car from holding idle and to the point where it'll shut off and only sputter when cranked.
CarlosJavier

Re: Engine runs, terrible stumbling / hesitation

Post by CarlosJavier »

Problem solved.

There was two issues within the AFM: the door was getting stuck within the housing and the electrical resistance between the sweeper and the swept surface was off ( 4000 Ohm between ECU connector terminals 7 and 8 ).

First I opened up the bottom lid of the AFM housing and cleaned everything inside, but the door would still stick. So I lightly sanded the insides of the AFM housing and the door edges. No more sticking door.

Then I opened the black box on top of the AFM and pressed the sweeper against the swept surface. Resistance dropped to 800 Ohm, well within spec.

Drove the car, no hesitation.

I did experience fuel pump cavitation twice. The car would start sputtering until it shut down; the fuel feed line would go flat. I'd prime the pump manually by swinging the AFM door, and the car would start and run normally. Annoying, but not as hard to diagnose/fix...
So Cal Mark

Re: Engine runs, terrible stumbling / hesitation

Post by So Cal Mark »

the trouble with sanding the afm sides is potential air leakage around the door. If you had to sand it, the door is warped.
On your fuel line collapsing, was it the hose between the pump and tank?
CarlosJavier

Re: Engine runs, terrible stumbling / hesitation

Post by CarlosJavier »

So Cal Mark wrote:the trouble with sanding the afm sides is potential air leakage around the door. If you had to sand it, the door is warped.
On your fuel line collapsing, was it the hose between the pump and tank?
The door would only stick if opened more than ~75%, so it just required very light sanding. I figured it might perhaps result in leaner AFRs but decided it was worth a shot considering the cost of a new AFM.

As for the fuel pump issue, all I know for now is that there was a pressuse drop at the injector rail (hoses could be squeezed flat) and that the fuel pump never stopped running. Next time it happens I'll check if the line between the tank and pump collapsed; thanks for the tip!
CarlosJavier

Re: Engine runs, terrible stumbling / hesitation

Post by CarlosJavier »

I celebrated too early...

Drove the car today for a little bit; it was fine at first but then the engine hesitation / stumbling came back and got progressively worse.

Compression test results:
1- 175
2- 175
3- 170
4- 160
So Cal Mark

Re: Engine runs, terrible stumbling / hesitation

Post by So Cal Mark »

I'd check the resistance of the coolant temp sensor throughout its' range. Your system may be going extremely lean as it warms up
CarlosJavier

Re: Engine runs, terrible stumbling / hesitation

Post by CarlosJavier »

Awesome tip; I'll do that as soon as I get back in town.
CarlosJavier

Re: Engine runs, terrible stumbling / hesitation

Post by CarlosJavier »

The last few times I started the car it would barely run, even when cold...

So I went back to square one and discovered that the fuel pump is barely moving gas. I haven't yet had time to check the voltage it's getting or if there are any restrictions. Hope to figure out this issue ASAP.
CarlosJavier

Re: Engine runs, terrible stumbling / hesitation

Post by CarlosJavier »

Finally found some time to work on the Fiat...

The fuel pump is buzzing normally, getting 12v, and there are no obstructions on the inlet line; but only a trickle of fuel comes out of the pump. Next I'll check with a vacuum/pressure pump if the hard feed line in the tank is obstructed. If it checks out I guess I'll have to fork out for a new fuel pump.
CarlosJavier

Re: Engine runs, terrible stumbling / hesitation

Post by CarlosJavier »

The Spider is up and running again!

All the fuel lines were clear but the inlet in the fuel pump itself was clogged. Some wire and acetone to that side of the fuel pump took care of the problem. Most of the clog must have built up from when I ran the car before thoroughly cleaning the gas tank, because the fuel I drained today looked pretty clean. Hopefully it'll stay that way.

Everything seems to be working good for the time being, so a brake job is next...
So Cal Mark

Re: Engine runs, terrible stumbling / hesitation

Post by So Cal Mark »

was the screen in the tank plugged?
CarlosJavier

Re: Engine runs, terrible stumbling / hesitation

Post by CarlosJavier »

There's a screen in the tank? When cleaning out the gas tank I did a search on the matter and came up with this:
pope wrote:No screen in F.I tanks, only big ass fuel filters the size of coconuts after the pump.
In any case, there are no obvious clogs. I tested the tank's hard lines with a vacuum/pressure pump and they checked out fine. Also, when I removed from the fuel pump the hose that comes from the tank, seven gallons of gas rushed out like if they were coming from a garden hose.
So Cal Mark

Re: Engine runs, terrible stumbling / hesitation

Post by So Cal Mark »

what was the pump inlet clogged with? If it was bits of rubber, then replace that hose from the tank to the pump
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