Got it running -- hesitates when cold

[VetteMed]

SO, after 3 months of working on it very intermittently, I fired up my '91 last night and drove it for the first time since I brought it home (bought it from an ebay seller in March).

When I drove it home initially, the secondaries were nonfunctional. During the course of changing the belt, the prior owner managed to damage the vacuum line running near the oil filter, as well as break the oil pressure sensor and oil temp sensor, rendering the secondaries inoperative. With the ignition on, the secondary vacuum pump ran continuously.

I pulled the plenum, installed new injectors from FIC, new GM coils, new wires, new plugs (gapped at 0.035). I removed the damaged section of vacuum line and spliced it back together.

SO, tonight I fired it up for the first time, once the rails primed it fired right up. The vacuum pump runs intermittently -- when it runs, it runs for only a second or two, then shuts off for an equal amount of time. This cycle repeates ad infinitum. The good news, is that the secondaries function -- WOW I still have some odds and ends to do, so I couldn't really enjoy it much, but it's definitely running better than when I brought it home.

However, during the first few minutes of driving, there's a bad hesitation/bogging from idle. It seemed to disappear when warmed up. As a side note, the car has a set of headers installed by a prior owner, no cats, and a chip from Z-Industries.

I ran around with my mityvac a little bit, and when I pull vacuum at the line from the pump to under the plenum, I can pull 10-15 mmHg but it drops approx 1mm/second thereafter. The same response is noted at the fitting on the driver's side between the intake runners, and the passenger side at the tube that runs from the coolant surge tank to under the plenum. When I pull vacuum on the tube between the runners on the passenger side, no drop in vacuum is observed. When I had the plenum off, the secondary actuators worked properly and held vacuum without dropping.

I know I still need to change the fuel filter, though I don't think that's a likely cause for a cold hesitation. Where should I look next?

Thanks!
Andrew

[batchman]

If you have a stock prom available, that's where I'd start. The performance chips need to compromise something in order to gain their HP advantage, drivability issues could be easy to introduce. I know the DRM chip in our car introduces some oddities (not that one, though).

Good luck,
- Jeff The Hapless Ford Guy

[VetteMed]

Originally Posted by batchman

If you have a stock prom available, that's where I'd start. The performance chips need to compromise something in order to gain their HP advantage, drivability issues could be easy to introduce. I know the DRM chip in our car introduces some oddities (not that one, though).

Good luck,
- Jeff The Hapless Ford Guy

Thanks for the info, certainly something to consider. I have another tuner chip here that I could swap in too, to see if there's any difference.

Originally Posted by VetteMed

However, during the first few minutes of driving, there's a bad hesitation/bogging from idle. It seemed to disappear when warmed up. As a side note, the car has a set of headers installed by a prior owner, no cats, and a chip from Z-Industries.

Andrew, sure you don't have a vacuum leak on the plenum somewhere that is sealing once it gets warm?

I ran around with my mityvac a little bit, and when I pull vacuum at the line from the pump to under the plenum, I can pull 10-15 mmHg but it drops approx 1mm/second thereafter. The same response is noted at the fitting on the driver's side between the intake runners, and the passenger side at the tube that runs from the coolant surge tank to under the plenum. When I pull vacuum on the tube between the runners on the passenger side, no drop in vacuum is observed. When I had the plenum off, the secondary actuators worked properly and held vacuum without dropping.

The vacuum line on the passenger side of the plenum is for the cruise control. It's a separate system.

[VetteMed]

Originally Posted by Jeffvette

Andrew, sure you don't have a vacuum leak on the plenum somewhere that is sealing once it gets warm?


The vacuum line on the passenger side of the plenum is for the cruise control. It's a separate system.

That was a thought of mine -- it acts like an old carbureted car with a small intake manifold gasket leak or carb gasket leak... I suppose I need to pull the plenum again.

I didn't use any sealant on the plenum gaskets when I reassembled (I was under the impression that none was needed). Hope that was correct.

Thanks for the info re: the passenger side vacuum line.

Originally Posted by VetteMed

That was a thought of mine -- it acts like an old carbureted car with a small intake manifold gasket leak or carb gasket leak... I suppose I need to pull the plenum again.

I didn't use any sealant on the plenum gaskets when I reassembled (I was under the impression that none was needed). Hope that was correct.

Thanks for the info re: the passenger side vacuum line.

No sealant required unless you have a warped manifold.

And drain plug is out the door to you.

[GregCrowe]

Mine has a slight hesitation at first startup and goes away as the car warms up. I'm almost positive it's from pulling the plenum so damn many times over my 4 year ordeal that ended up being the coils. I'm pretty sure I need to replace the plenum gaskets. You many have the same issue. I haven't looked into where to get gaskets yet.

[jonszr1]

go to jerry gaskets , jerry is a great guy that owns i believe 2 zr1s plus his product is first rate and prices decent too .

[VetteMed]

Gaskets were new... however, I pulled the plenum again and found that the secondary solenoid is leaking vacuum. SO I'm going to replace that and see what happens.

[VetteMed]

Another bit of info -- today during my running-hot adventure, the hesitation/stumble did NOT go away after warmup. It remained consistently rough at low rpm (smoothed out at around 4K or so). I installed all new injectors from FIC when I pulled the plenum. The car had new fuel pumps installed a couple of years ago by the PO. New GM coils, wires, and plugs.