ttruelove

Hi Everyone,
I have read a TON of posts like mine and I'm trying as many of the suggestions as possible. I'm getting to a point where I'm about to have to start troubleshooting higher dollar items and I want to be right. I don't want to just randomly replace parts. Here's the scoop:

I bought this 91 RS with a 350 swap. I do not know if the installed throttle body and other sensors/injectors are for the 350 or original stock 305. If someone could help with that, I'd be very grateful! The manifold was replaced, but it appears stock as well. I don't really 'know' this car, but I am 'learning its quirks.' The car usually starts up fine after about 4-5 cranks and sounds/runs good in Park/Neutral. I do hear a loud air sucking noise from the throttle body (which I assume is normal). The air noise calms down after a few minutes. I can leave it in Park and it'll hum forever. However, when I put it in Drive, the RPMs drop pretty low and it'll drive fine for a while.

I can drive it about 10-15 miles and when I pull up to a stop light, the car may or may not die. When I have my foot on the brake, the car wants to surge a bit. When I let my foot off the brake, the car will sometimes stay alive and drive. Other times, the car will die as I have the brake pressed. It only happens at low rpms and only when coming to a stop or sitting in Drive/Reverse for a while. The issue is completely random and no codes are being thrown when it happens.

Most times, when the car dies, it starts up just fine (2-3 cranks). However, this morning was different. I drove it to work and it died at a light about 12 miles away from home. I put it in Park and started the car right back up. Then, when I pulled into my work's parking lot, I took a corner slow and the car died. When I tried to crank it, the cranks came VERY slow and it had a very hard time turning over. I was concerned that the car wouldn't turn over at all. Once it did (4-5 times cranking slow), it fired up and I was able to put it in Reverse and park it. I can't trust the car to get me anywhere and I'm looking for any help you can provide. Also, while I was driving this morning, the Service Engine Light came on for about 2 minutes and then went away.

I've read that a lot of things can cause this issue, but let me start with what I've done so far (based on suggestions from other posts).

- Replaced Throttle Position Sensor
- Cleaned AIC (it was soooooooo dirty. Black. I really don't know if it's actually working)
- Replaced Fuel Filter (looks like it was replaced recently, but I put a new one on just to make sure)
- I've checked many of the vacuum hoses, but I'm not sure which ones specifically. I did find a small severed hard line on the passenger's side leading from the valve cover. I patched it with some rubber shrink tubing, but I don't know if helped.

I've read that the following can cause this issue:
- Bad distributor/coil/cap/wires/plugs (the cap/wires look new, but I don't know about the coil)
- Bad MAP sensor
- Bad Alternator (mine looks recently replaced)
- Bad relays
- Bad fuel pump
- Vacuum leaks (brake booster, specifically)
- Wrong size throttle body (I find this unlikely for my situation, but you can tell me)

I'm sure there are more I missed, but it gives me a good start. 1) The bad distributor seems logical since the car is having random trouble turning over when it dies. 2) The MAP sensor is a somewhat cheap part to replace and seems possible. 3) Vacuum leaks seem definitely possible, but I can't 'see' any issues with them. I've followed the lines and they all seem clamped with no visible wear. 4) Could a bad fuel pump cause this to happen only when it's in gear? I checked the injectors and they seem to be flowing alright in Park.

I don't have a code reader. I have limited knowledge on using a voltmeter (would need hand-holding, sorry).

Any help would be greatly appreciated.

Kaya

Not too sure if you have fixed this or not...... I am battling the same thing currently, although, it appears mine is less severe than yours. It really doesnt happen that often with mine.

Like you I replaced everything with very little success. I wish instead of buying all the various parts that didnt seem to fix anything, I would have listened to the advice I first got about my issue.

Buy a ALDL cable. I just got mine in the mail, and although I havent had any time to check it out, Im willing to bet I will get a much better idea once I do.


Good Luck!

ttruelove

Following up: A leak-down test proved the injectors were not holding pressure. Fuel pressure was fine (10psi), but the TBI injectors were both leaking and one was installed incorrectly. Replaced with new parts and upped the idle. Issue is better, but not 100% resolved. When the car is hot, a shift from drive to reverse bogs the car down and it 'wants' to die. The higher idle helps the computer compensate, but I don't know why the bogged issue is occurring.

I read on a previous post that a vacuum leak in the break booster line might cause this. Is that true? Looking for any other ideas. Thanks.

Ghettobird52

OK, you say a 350 TBI swap, but was it a 350 ecm swap as well, or tune involved? When i got my 350 swapped in, at first it was just the motor with the 305 throttle body and ecm still running it.

Do you know you have a 350 ECM or tune?

Do you know you have 350 injectors?

Thats where i'd start...

ttruelove

You actually read my mind. To be 100% honest, I don't know if the ECM was replaced/updated. I did buy a chip/writer from Moates, but I'm not sure which bin file to use. I tried one they had on the Moates site from a Caprice, but the car bogged down to almost nothing when driving. The injectors are new 305s. I had a shop replace them once they did the leak-down test. I'm still learning the conversion dos and don'ts.

So, I've read several posts on this and my next steps are replacing injectors (350 from a Caprice), better fuel pump to compensate, and getting help with the tune (anyone??? ). Thanks for the suggestion Ghettobird52.

ttruelove

On another note, could a bad distributor cause this issue? In Park, the idle is fine, but as soon as it's dropped in gear, the RPMs drop and the engine starts to surge? Just curious...

Schurkey

If I had problems with slow cranking, I'd be looking at the battery, starter motor, alternator, and the wiring harness that connects them all.

There's nothing in the computer/sensor/actuator system that will cause slow cranking.

Low system voltage can cause other problems beyond slow cranking.

Get a scan tool, discover and write down all stored codes, CLEAR THEM, and then examine the data stream.

See what codes come back.

ttruelove

YES! Great information, thank you. I was going to replace the alternator and battery anyway and this just confirms it. I remember an old trick with my Mustang when it had trouble starting and that was to bang on the starter with a hammer (as long as it had power). I guess gunk builds up over time and creates problems with the starter? That's what I remember, anyway. Might as well replace the starter while I'm at it. At least I'll know it's new.

I'll definitely get the scanner. Thanks!

Schurkey

No. Bushings and brushes wear, the starter has high internal friction and poor electrical contact. Whacking it with a hammer "adjusts" the position of the parts inside, leading to better electrical contact and perhaps less friction.

How about TESTING the electrical system, and only replacing the parts (including wires and switches) that actually need it?